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RingEX is a cloud-based business communications solution that offers tools for Messaging, Video and Phone. Core features of the solution include conferencing, auto-recording and unlimited long-distance and local calling. RingEX ...Read more about RingEX

4.2 (1195 reviews)

30 recommendations


Nextiva brings customers and teams closer by connecting conversations in one AI-powered platform. We connect businesses to all of their conversations, helping them understand their customers in real-time to create personalized exp...Read more about Nextiva

4.6 (888 reviews)

2 recommendations


Designed as a hybrid call center solution, SpitFire Enterprise utilizes powerful dialing tools to assist call centers in the telemarketing, financial services, healthcare, and education industries launch blended inbound and outbou...Read more about SpitFire

4.5 (22 reviews)

2 recommendations

net2phone Canada

net2phone Canada is a cloud-based business phone service provider, helping Canadian organizations drive business performance through powerful uCaaS tools. Trusted by clients and partners across the country, net2phone Canada has di...Read more about net2phone Canada

4.6 (10 reviews)

2 recommendations

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Dialpad is an AI-powered cloud communication platform and business phone system that provides organizations with the latest in VoIP telephony technology to keep teams connected and productive. The platform comprises three integrat...Read more about Dialpad

4.3 (522 reviews)

1 recommendations


OnSIP Hosted OnSIP Hosted VoIP by Junction Networks is a cloud-based phone solution that offers businesses across various industry verticals PBX feature such as auto attendants and voicemail-to-email. Unified communications (...Read more about OnSIP

4.6 (49 reviews)

1 recommendations


Talkdesk is a cloud-based call center solution that helps businesses improve customer satisfaction while simultaneously reducing customer support costs. It uses interactive voice response (IVR), automatic call distribution (ACD) a...Read more about Talkdesk is a cloud-based internet phone service solution designed to help businesses manage local, mobile and toll-free number portability across Canada and United States. Key features include voicemail, call waiting, caller ID, D...Read more about

BlueJeans Meetings

BlueJeans is a cloud-based video conferencing solution that helps mid to large size businesses with text chat, content sharing, multiple participants and meeting recording. Blue Jeans delivers services through products including B...Read more about BlueJeans Meetings


CallHippo is a cloud-based call center solution that helps mid to large size businesses with workflow automation and virtual telephony. The platform enables users to give detailed analysis to customers through call recordings. Cal...Read more about CallHippo


Organizations around the world are striving to deliver the ultimate customer experience, strengthen brand value, and boost efficiency. NICE CXone empowers brands to achieve these objectives on one interaction-centric platform with...Read more about NICE CXone


Five9 is an all-in-one cloud contact center solution for inbound, outbound, blended and omnichannel contact centers world-wide. Powered by Practical AI, Five9 enables agents to provide customer experiences across phone, emai...Read more about Five9


Twilio is the worlds leading cloud communications platform that enables businesses to build, scale, and operate their own customized communication solutions. The platform is designed to be customizable and easy to use, which means...Read more about Twilio

Vonage Business Communications

Vonage Business Communications is a good option for small-, medium-, and enterprise-level businesses that need a scalable, easy-to-use VoIP platform with premium voice clarity and a wide array of solutions for specific business ne...Read more about Vonage Business Communications

MiVoice Business Solution

MiCloud Connect (formerly ShoreTel) is a cloud-based VoIP phone system that offers collaboration and conferencing tools to enable information flow within the organization. The solution enables system administrators to manage use...Read more about MiVoice Business Solution

Kixie PowerCall

Welcome to Kixie, the sales engagement platform that revolutionizes the way you do business. Designed to supercharge your sales performance, Kixie offers ultra-reliable and easily automated calling and texting features that seamle...Read more about Kixie PowerCall


Convoso’s all-in-one contact center platform generates and converts leads faster using multiple communication channels. The cloud-based solution offers multiple dialing modes (predictive/power/preview) along with two-way texting, ...Read more about Convoso

Zoom Phone

Zoom Phone is a VoIP solution designed to help businesses in education, finance, healthcare and government sectors connect and communicate with team members on a unified interface. It offers a central administrator portal, allowin...Read more about Zoom Phone


Acefone is a VoIP solution designed to help businesses in construction, retail, real estate, healthcare, finance, education and other sectors manage missed client calls, define ring strategies for different phone numbers and contr...Read more about Acefone


Bandwidth is a modern communications platform giving you access to voice, messaging, and 9-1-1 from the cloud. They offer you the power and reliability of a fully-owned nationwide all-IP voice network paired with a platform of ent...Read more about Bandwidth

Buyers Guide

Last Updated: March 16, 2023

SIP trunking is a network service that allows businesses to connect VoIP phone systems to the traditional phone network. While this service is generally included with “per-user” licenses for cloud-based phone systems, if you have an on-premise system you’ll most likely need a SIP provider to connect calls with it.

We’ll explain what SIP trunking is, why you need it, and the primary considerations that you should factor in when deciding on a provider.

Here's what we'll cover:

What Is the Session Initiation Protocol (SIP)?

What Is a SIP Trunk

Benefits of SIP Trunking

Features of SIP Trunking Service

Important Considerations for Buyers

What Is the Session Initiation Protocol (SIP)?

Imagine the Internet as a vast network of roads and highways. On it are many types of vehicles, delivering various-sized packages (or packets, to use the technical term) containing a variety of information at a wide range of speeds. Most Internet users only ever see the contents of the vehicles traveling on the roads.

But like real roadways, the Internet has many layers below the surface. These lower layers support—and determine the quality of—the layers above them. Each layer has its own specific set of languages used to communicate within that layer. These languages are called “protocols.”

One of the lowest layers supporting the Internet is called the link layer. Within the link layer, link layer protocols organize groups of local connections into large networks. At the top, just under the surface, is the application layer. The application layer determines the quality of the “road surface” directly above it. SIP is a protocol that operates within the application layer.

Different application layer protocols are used to create different surfaces. The Internet has a wide variety of road surfaces, some favoring certain types of traffic over others: Compare a heavily loaded truck slowly climbing the rough road surface of a hill to a sports car zipping down smooth concrete. Some surfaces work better for certain purposes.

VoIP—phone conversations over the Internet—are one very particular type of traffic. They require audio to be delivered consistently with minimal delay. SIP is an application layer protocol designed to provide a very certain type of road surface for this special traffic. SIP initiates the sessions (or connections) that prepare the network to best meet the specific needs of a real-time audio connection over the Internet. 

We offer a fuller explanation in our article on the SIP protocol.

What Is a SIP Trunk?

We’ve mentioned that SIP trunking is a network service. It’s necessary because VoIP calls travel over the internet rather than the traditional phone network. This means that you need some way to connect calls between the internet and the traditional phone network, as otherwise you’d only be able to call other users with VoIP.

SIP trunking is the solution to this problem.

A SIP trunk isn’t a phone “line.” Instead, you connect to your SIP provider’s network using your internet connection. Your SIP provider utilizes specialized technologies to create a bridge between the internet and the traditional phone network, thereby connecting calls between these networks.

In some cases, you can get SIP trunking from the same company that provides your internet service, but in other cases you may need to use separate providers for SIP trunking service and internet service.

If you don’t have SIP trunking, you won’t be able to do much with a business VoIP phone system aside from admiring the server hardware. And while the traditional phone network is dying in the United States, it’s alive and well across the globe, so the need for SIP trunking isn’t going to disappear anytime soon.

Benefits of SIP Trunking

SIP trunking can cut costs in the following ways:

Lower trunking costs. Combined voice and data connections have traditionally been provided by telecoms in the form of Integrated Services Digital Network (ISDN) lines, also known as trunks. ISDN trunks come with fixed quantities of lines per trunk (for example, T1 trunks have 23 lines each). If a business needs even one additional line, it must purchase another entire trunk—meaning businesses often pay for unused capacity. SIP trunks are comprised of bandwidth, a flexible commodity, and their capacity can be increased on the fly.

Lower equipment expenses. Offices without SIP trunks use media gateways to translate between the analog PSTN and the office’s internal digital traffic. Offices using SIP trunks don’t need this expensive piece of equipment because their traffic bypasses the PSTN. A SIP trunk also makes it more practical for an office to use a cloud-based phone system rather than purchasing and maintaining an in-office hardware version.

Lower calling charges. Traditionally, and under many existing calling plans, charges are calculated based on the physical length of the PSTN line required to complete a call. Calls made via SIP trunks bypass most of the PSTN and so avoid many long-distance charges.

SIP providers also frequently offer better rates on international calls than traditional phone service providers, and most importantly, intra-office calls and calls between branch offices are frequently free when connected via SIP trunking.

SIP trunking also has a number of technological advantages:

More communications options. A SIP trunk extends, in a virtual sense, an office’s internal network to the outside world—letting remote offices, mobile employees and customers worldwide interact as if they were in the same building. Click-to-call website options for customers, application sharing and simple call transferring for work-at-home or abroad employees, for example, can all be done more easily with SIP trunks.

Meanwhile, SIP client apps can facilitate communication via a mobile device over an Internet connection, with capabilities such as voicemail, SMS messaging and more.

Features of SIP Trunking Service

The following table shows some common features of SIP trunking service:


Traditionally, if you needed more calling capacity on your phone lines, your provider took a month or longer to deliver it. SIP providers, on the other hand, can scale capacity up and down in as little as 24 hours, a vital cost-saving feature for businesses with fluctuating call volume.

Port sharing

If you have multiple sites, and one site maxes out its number of simultaneous calls, you can connect calls via open ports at one of your other sites. Again, this is a vital cost-saver for call centers.

Disaster recovery

In case of a service outage at one of your sites, your SIP provider can forward calls made to critical phone numbers.

911 and E.911 support

Your SIP provider can support 911 calls by providing location information with the call. Without E.911 support, VoIP isn’t compatible with emergency services.

MPLS support

Multi-protocol label switching is a networking technology that allows information to be routed within a network using path labels rather than the network protocols we discussed above. This offers performance, reliability and security benefits, and some businesses prefer to have SIP trunks delivered over MPLS. You’ll need to make sure your provider offers both MPLS and SIP trunking services, however.

Important Considerations for Buyers

Phone system deployment type. Whether you have an on-premise or cloud-based system is the most important factor affecting your approach to SIP trunking. Businesses with cloud systems may not even need a separate SIP provider.

In most cases, SIP trunking is included with “unlimited” user licenses for cloud systems. The vendor generally doesn’t even mention that SIP trunking is included, given the complex nature of the service. Other cloud vendors provide “metered” SIP trunking - i.e., SIP trunking service priced using per-minute rates. If you have an on-premise phone system, however, you’ll need to search for a SIP provider, as generally vendors of on-premise systems don’t offer SIP trunking.

**Phone system brand.** Different phone system vendors have implemented the SIP protocol in their systems in different ways. This means that not all SIP providers will be compatible with your specific brand of phone system. Check with your phone system vendor, as many list the SIP providers that their systems support. You also need to ask your SIP provider about support for specific features and devices such as FAX machines.

Connection type. A SIP trunk can be provided over most any high-speed Internet connection. The cheapest option uses an existing broadband Internet connection from a standard Internet service provider (ISP), and simply has the ITSP service share it. Call quality and capacity may be reduced because both data and voice are sharing the connection. A more expensive—but better—option is to lease a line from an ITSP and dedicate it solely to your SIP traffic. Giving your SIP traffic its own line helps improve call quality and reliability. Not all SIP providers offer internet service, but many partner with ISPs.

Quality of service and service level agreements. Quality of Service (QoS) and Service Level Agreements (SLAs) are common components of the contracts businesses sign with their ITSP. The QoS agreement defines (in technical terms regarding IP traffic’s delay, jitter and packet loss) the acceptable average quality of a voice call. The SLA, on the other hand, describes exactly how much "downtime" a service provider is allowed, what constitutes downtime and how quickly network issues must be resolved.

Trial period. ITSPs recognize that it can be difficult for a business to commit to a new phone system without experiencing it firsthand. For this reason, many now offer full or limited trial periods. The trials can be adopted on a small scale to make sure everything is functioning as expected before making the switch. Make sure to test any devices such as FAX machines that are connected to the phone network as well as the actual features of your phones.

911 dialing. The FCC has mandated that all “interconnected” VoIP services provide functionality to connect 911 calls to the appropriate local emergency services authority. But not all ITSPs fall under this mandate. If a business plans to eliminate their PSTN service entirely, then this needs to be considered. Maintaining a single PSTN line for emergencies is a viable option.

Firewall and security. If not properly managed, SIP trunks can expose an office network to external privacy and security threats. Traditionally, offices have used standard firewalls to keep out unwanted IP traffic, but SIP traffic does not always travel smoothly through firewalls. Other security options, like Virtual Private Networks (VPNs) and session border controllers, are increasingly common.