Top Considerations for Selecting and Implementing a SIP Provider (Part Two)

By: on December 8, 2015

In the first part of Software Advice’s report on how information technology decision-makers select SIP providers, we highlighted the top economic and operational benefits of SIP trunking.

Here in part two, we focus on issues related to SIP trunking installation and maintenance, such as security options and service-level agreements (SLAs).

Based on our survey of IT decision-makers about their SIP trunking deployments and provider relationships, this report will identify the primary technical factors businesses should consider when choosing a provider.

Key Findings

  1. Thirty-two percent of respondents using on-premise phone systems with SIP trunking say they “rarely” experience service outages, compared to just 18 percent of those using cloud-based systems.
  2. Twenty-eight percent of respondents with on-premise systems say that all PBX features are supported, while 36 percent of respondents with cloud-based systems say the same.
  3. Eighty-nine percent say providers “very often” or “always” deliver on SLAs related to service recovery after outages; 88 percent say the same for SLAs related to call quality.
  4. Seventy-two percent of respondents use a session border controller supplied and managed by their SIP trunking provider, with 40 percent using this device specifically for security purposes.
  5. Transport layer security (TLS), which is used to prevent denial-of-service (DOS) attacks, is the most common security measure for SIP trunking: 58 percent of our overall sample say they use it.

Considerations for Choosing a SIP Provider

The process of selecting a SIP provider can involve a bewildering array of technical details. For example, some providers ask prospective clients complex questions about the different types of SIP requests supported by their private branch exchange (PBX) systems.

A PBX system is the foundation of a business phone system. It connects extensions within a corporate directory to a telecommunications network, such as the traditional telephone network or the Internet.

An Internet-enabled PBX system, designed for use with SIP trunking (see part one for a definition of this service), is known as an “IP PBX,” short for Internet Protocol PBX.

Different types of phone systems offer different benefits when used in conjunction with SIP trunking. Additionally, IT decision-makers who evaluate SIP providers are confronted with a wealth of security options, SLAs and managed hardware plans.

While part one of this report focused on the benefits of SIP trunking, in part two, we’ll discuss which options are most popular among our respondents, and offer the most concrete business value for companies in the market for a provider.

On-Premise PBXs More Reliable Than Cloud-Based

Cloud-based PBX systems have gradually been gaining traction over the past decade—a trend supported by the findings published in the first part of this report.

For example, 53 percent of the organizations in our sample have fully consolidated their SIP trunks, phone systems and broadband Internet connections in a single, cloud-based provider.

As we explain in part one, there are a number of reasons for consolidating providers and adopting a cloud-based solution.

However, our findings also indicate that there are substantial benefits of adopting an on-premise solution in conjunction with SIP trunking. In particular, organizations can improve the reliability of their phone systems by choosing an on-premise PBX.

While 60 percent of respondents with cloud-based PBX systems report that service outages happen “often” or “very often,” only 53 percent of respondents with on-premise systems report this level of frequency.

Moreover, a combined 35 percent of respondents with on-premise systems report that outages are rare or non-existent in their organizations, while only 21 percent of those with cloud systems say the same.

Outage Frequency, by Type of PBX System

Outage Frequency, by Type of PBX System

There are a number of technological reasons behind these reported variations in outage frequency. For instance, many IP PBX systems offer backup connectivity options that allow businesses to route calls over the traditional phone network when their Internet connections go down.

Additionally, the performance of cloud PBX applications can be affected by issues with the public and private networks over which employees access them.

On-premise servers, on the other hand, allow for more direct connections to PBX applications. (We explain how network design can affect application performance in more detail here.)

As such, organizations for which constant phone service is mission-critical should consider using an on-premise phone system in conjunction with SIP trunking.

SIP Trunks Offer Easy Configuration

Although businesses can increase reliability by using an on-premise PBX in conjunction with SIP trunking, we find a nearly even adoption rate for both types of systems among our sample.

Fifty-three percent say their primary PBX is an on-premise system, whereas 47 percent are primarily relying on a cloud-based solution.

Indeed, our findings show that cloud solutions offer numerous benefits beyond reliability. In particular, SIP trunks support a greater range of PBX features (e.g., specialized modes of call routing) and functionality when used with cloud-based rather than on-premise systems.

Cloud users can also avoid some of the compatibility issues that can plague users of on-premise systems.

We find that, on average, SIP providers support a greater range of features for cloud-based PBX systems than they do for on-premise PBX systems.

This difference is due in part to the fact that 63 percent of the respondents in our sample have consolidated their cloud PBX provider with their SIP trunking provider, as noted in part one of this report. This consolidation ensures native interoperability between the PBX system and the SIP trunk, since the same provider handles both services.

PBX Features Supported by SIP Trunking: Cloud vs. On-Premise
Difficulty Configuring PBX for SIP Provider: Cloud vs. On-Premise

With on-premise systems, however, the SIP trunking provider has to ensure that its service can support proprietary PBX software and hardware from a third-party vendor. Such multivendor environments can suffer from compatibility problems, including lack of support for any number of PBX features.

Thus, you should verify that any potential SIP provider supports all functionality that is mission-critical to your business.

When it comes to configuration, our respondents have more difficulty configuring on-premise systems than cloud systems, most likely because of the proprietary hardware involved in this setup.

Indeed, 4 percent of the on-premise group say their SIP provider and their existing PBX systems are completely incompatible, forcing them to look for another SIP provider.

None of the respondents using cloud PBX systems report experiencing such severe compatibility problems—even those with different SIP trunking and PBX providers.

This finding suggests that, if you’re building a system from the ground up, you may want to explore a cloud-based PBX system (particularly one bundled with SIP trunking service) in order to eliminate configuration and compatibility headaches.

Vast Majority of Respondents Satisfied With SLAs

Another way to minimize configuration and maintenance issues is to select a SIP provider that offers the SLAs your business model requires. SLAs are contracts that formally define the levels of service businesses can expect from their SIP providers.

The most important types of SIP trunking SLAs relate to:

Installation: Guarantees that installation will be complete within a specified time frame, thus ensuring that the switch will result in minimal downtime.

Mean time to recovery (MTTR): Guarantees that the provider will take, on average, a fixed amount of time to get service back up after an outage.

Mean opinion score (MOS): A rating scale for the subjective experience of audio quality. While the scale is based on human experience, there are software applications that can monitor technical factors related to network performance that impact call quality, and assign scores correspondingly.

R-Factor score: An alternative, and somewhat more sophisticated, scale for measuring call quality. SLAs relating to MOS and R-Factor guarantee that providers will deliver an average level of call quality determined by a MOS or R-Factor score.

The following chart shows the levels of prevalence that we found for these different SLA types:

Prevalence of SLAs

Prevalence of SLAs

We find a nearly equal level of occurrence for all SLA types we asked about, though installation SLAs slightly edge out MTTR and call-quality SLAs.

Moreover, we found that on average, most providers adhere to the ongoing MTTR and MOS/R-Factor SLAs that they offer:

Frequency of Providers’ Adherence to SLAs

Frequency of Providers’ Adherence to SLAs

Overall, our data suggests that SLAs are a highly effective way to maintain consistent uptime and call quality.

A combined 89 percent of respondents say their providers “very often” or “always” deliver on MTTR SLAs, while a combined 88 percent say the same for MOS/R-Factor SLAs. (None of our respondents say their providers “rarely” deliver on SLAs, even though we offered this option.)

A Vast Majority Have Managed Routers and Session Border Controllers

Another way to streamline the SIP trunking deployment process is to choose a managed hardware plan from your SIP provider. This may include a managed router and/or a session border controller (SBC).

With a managed router, the provider installs, configures and monitors the customer’s router to help them maintain constant uptime.

A SBC, on the other hand, is deployed at the border between a business’s private network and a service provider’s network (in this case, the SIP trunking provider’s network).

The SBC controls all inbound and outbound traffic based on the SIP protocol, including multimedia and voice traffic.

SBCs have a number of functions related to VoIP security. For instance, they hide the structure of corporate networks (i.e., the connections between routers, switches, servers etc.) from would-be hackers, and help ensure that on-premise equipment from different vendors works together smoothly.

Among the IT decision-makers in our sample, we find high adoption rates for both managed-router and SBC plans.

However, managed routers are somewhat more popular—probably because most businesses require routers to connect to service providers and/or to support their virtual private networks (VPNs; a security measure used with SIP trunking that we’ll discuss in more detail below).

Adoption of Managed Hardware Plans

Adoption of Managed Hardware Plans

Many smaller businesses can also get by without deploying a SBC. Small firms tend to be less of a target for the DOS attacks (a cyberattack to take down an essential network service, such as Internet access) that SBCs help prevent.

They also have less substantial IT investments in on-premise equipment—as well as fewer locations with fewer PBX systems—than enterprises do. Smaller companies thus have less of a need for SBCs that ensure interoperability between PBX systems from different vendors.

That said, our sample reports near-universal adoption of some type of managed hardware plan (only 2 percent of respondents don’t have one). This makes sense: Most organizations stand to benefit from at least one of these plans.

While smaller businesses might need a third party to manage their routers, larger businesses with greater IT resources also have a greater need to secure their networks from outside threats.

Additionally, enterprises have more sites to connect together in a unified phone system—frequently including equipment from multiple vendors.

The question, then, is not whether a managed hardware plan is necessary, but which kind of plan will best meet your company’s needs.

TLS Is Top Solution for Securing SIP Trunks

Finally, we asked respondents about the security measures their businesses take. While overall, SBCs have been widely adopted by the respondents in our sample, they’re not quite as popular as other methods when it comes to securing SIP trunks. Here’s a brief rundown of the security measures we’ll cover:

TLS: A protocol for encrypting IP communications. TLS is used in Web browsing for purposes such as secure transmission of payment card information, and also for VoIP security.

TLS encrypts the SIP requests exchanged between clients (i.e., businesses) and servers (i.e., SIP trunking providers). This helps to prevent DOS attacks, as SIP requests control VoIP calls and other communication sessions, such as video conferences.

VPNs: A type of private network. A truly private network is one where connections between devices eliminate the need for data to travel over the Internet, which is a public network. Private networks can be created by setting up direct wireless or hard-wired connections between devices.

In some cases, geographic and network-design considerations make direct connections impossible, so only a “virtually” private network can be deployed. Here, data in the VPN must be routed over the public Internet, and security measures must be put in place to segment the VPN from the rest of the Web.

There are a number of different types of VPNs, but here we’ll focus on:

    • Internet Protocol Security (IPsec) VPNs: A type of VPN commonly used with VoIP to enable secure voice calling and video conferencing. IPsec VPNs encrypt the data packets that communication sessions consist of (as opposed to TLS, which encrypts the SIP requests that control how packets are sent).Thus, IPsec VPNs help prevent third-party interception of communication sessions, whereas TLS is primarily useful in staving off DOS attacks.
    • Multi-protocol Label Switching (MPLS) VPNs: A type of VPN supported by an MPLS network. In MPLS networks, data packets are assigned labels that control how they are routed between network nodes (e.g., routers and switches).MPLS can thus help ensure that data is transmitted efficiently in complex networks by using the shortest possible pathway. While MPLS is frequently used to guarantee a consistent level of performance for mission-critical services such as VoIP, it can also be used to set up VPNs between geographically separated sites.

The following chart shows the levels of adoption for these different security measures:

Adoption of SIP Trunking Security Measures

Adoption of SIP Trunking Security Measures

While we’ve already seen that 72 percent of our sample has adopted managed SBC plans, the above chart shows that only 40 percent are specifically using SBCs for security purposes, such as cloaking network structure from hackers.

We can infer that in most of the other deployments involving managed SBCs, they have been implemented to ensure interoperability between VoIP equipment from different manufacturers (their other major function).

Finally, the above chart shows that currently available VoIP security measures are still far from being universally adopted. Businesses must ensure that potential providers support the security options they need.


Our analysis of IT decision-makers’ relationships with their SIP trunking providers reveals the following key considerations for provider selection:

On-premise systems have fewer SIP provider outages than cloud systems. If you’ve already invested in on-premise equipment, you may want to preserve it when selecting a new provider, rather than shifting to a cloud-based solution.

Cloud systems are more compatible with SIP providers than on-premise systems. A given on-premise IP PBX system may not be compatible with some SIP trunking providers. If you’re building a system from the ground up, you may want to go the cloud route to ensure full use of your phone system’s functionality.

If you already have an on-premise system, do careful research to ensure that your new SIP provider can support it.

SLAs deliver on uptime, call-quality and installation guarantees. Not a single respondent says their provider “rarely” delivers on SLAs. If constant uptime or consistent call quality are mission-critical for your business, you should find a provider that guarantees your level of service in these areas.

Installation SLAs can also help ease the process of switching from old providers to new.

Managed hardware plans are smart investments. Managed routers can decrease the work of troubleshooting network-performance issues. Managed SBCs can ensure interoperability between PBX systems in multi-vendor environments and help prevent devastating DOS attacks.

While managed router plans can assist small businesses without extensive IT resources, managed SBC plans can help with enterprise security and networking problems.

TLS is the most widely adopted VoIP security mechanism. While provider offerings around VoIP security are still far from standard, ensuring that your provider supports SIP signaling over TLS is a good place to start.

More advanced security mechanisms (such as SBCs) may not be appropriate for all business models, though they’ve still been adopted by nearly half of our sample.

To find the data in this report, we conducted a five-day survey of 30 questions, and gathered 208 unique responses from random IT decision-makers. Responses to the survey were collected by our research partner, Lightspeed GMI, with whom Software Advice has no financial relationship. All responses were obtained in a fair and anonymous manner.

Results are representative of our survey sample, not necessarily the population as a whole. Expert commentary solely represents the views of the individual. Chart values are rounded to the nearest whole number.

If you have comments or would like to obtain access to any of the charts above, please contact


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