Explore 6 SIP trunk providers with these comparison points

Explore these six SIP vendors and the pros and cons that they each introduce to your company's telecommunications system.

SIP trunk providers can make for a difficult IT buying decision. The Session Initiation Protocol is a signaling and control protocol that transports real-time multimedia communications across IP networks. While all SIP providers offer similar trunking services based on well-established standards, there are important differences when it comes to their architecture capabilities, partnership commitments, technical support and financial stability. Prospective buyers must be well versed in how SIP trunking works and the benefits it has ushered into the enterprise over the years. Below are six of the top SIP trunk providers and a summary of their strengths and weaknesses. Using this information, readers will gain a better understanding of the subtle -- yet critical -- differences that may make one provider more suitable over another.

SIP impact on the enterprise

Voice, video and IM are common multimedia applications that use SIP for transport. SIP was standardized in 1999 by the Internet Engineering Task Force as RFC (Request for Comments) 2543. Soon after, carriers began offering SIP trunking services as an alternative way to connect on-premises business phone systems to the public switched telephone network (PSTN). Prior to SIP, the only way to connect business voice over IP phone systems was to use a VoIP gateway as an intermediary point that converted VoIP packets to legacy PSTN analog or digital protocols. Yet, with SIP, no gateway is necessary as everything is based on IP.

SIP, UCaaS and hybrid options

With SIP around for over 20 years, it's not exactly the newest kid on the block when it comes to unified communications (UC) technologies. That designation currently resides with UC as a service (UCaaS) offerings. UCaaS uses a public cloud architecture model to provide business customers with voice, video and other UC applications without the need for any on-premises hardware or software at the customer location. The cloud-based model is appealing to those that either haven't invested in on-premises UC equipment or have equipment that is nearing an end-of-life status.

That said, UCaaS products do not offer all the features and functionality that traditional on-premises platforms possess. For large businesses that have complex VoIP architectures and massive investments in on-premises UC equipment, UCaaS is not regarded as a functionally or financially viable option by industry analysts. Thus, it's larger organizations that are sticking with SIP trunks and on-premises equipment, while the SMB market is beginning to migrate more toward UCaaS.

Some SIP trunk providers that are also UCaaS providers do offer hybrid services that enable some voice, video and IM services to be on premises, while others migrate into a UCaaS cloud. If that's something that's appealing to your organization, you should seek out partners that offer hybrid or UCaaS services that are compatible with your current on-premises UC platform.


As the second largest telecommunications company in the United States and third largest in the world, AT&T is a powerhouse in the UC world. The publicly traded company has a market cap of $280 billion and operates all over the world, according to Macrotrends.

AT&T's IP Flexible Reach service is a unique SIP trunk offering in that it bundles SIP trunk services with internet broadband for businesses. The bundled pricing is attractive as both services are necessary for many businesses, and bundling products together in a single contract can save money over time. Because AT&T is also a major WAN circuit provider, it can also offer dedicated SIP trunk WAN connections to guarantee call quality as voice packets move between the customer network and the PSTN.

Typical AT&T SIP trunk customers have used AT&T analog and digital voice services in the past and wish to migrate to newer, SIP-based technologies. Because these businesses were previously customers, they do not have to worry about porting their telephone numbers over to a new carrier -- a process that can be time-consuming otherwise.

Large businesses with multiple remote sites around the country and throughout the world will tend to use larger telecommunications companies, like AT&T, because of their massive global presence. These businesses also tend to deploy distributed SIP trunking models, and larger SIP trunk providers are better suited to provide the level of call quality and reliability for SIP trunks in most geographical locations.

Some drawbacks of AT&T SIP services are subpar customer service, antiquated self-service customer portals and a lack of innovative add-on UC services that some businesses may be seeking, according to reviews from ConsumerAffairs and Trustpilot.


CenturyLink has always been a major player among SIP trunk providers, and once the company completed its acquisition of Level 3 in late 2017, it became the biggest SIP provider in the country. Despite having a market cap of $15 billion, much lower than both AT&T and Verizon, CenturyLink's purchase of Level 3 positions it as a global force in terms of SIP trunk market share.

CenturyLink has similar SIP trunking features and business benefits when compared to AT&T and Verizon. Its IQ SIP Trunk offering covers most of the eastern, southern and western parts of the continental U.S. -- with only a few gaps in the northern and central parts of the country.

Another benefit of the Level 3 acquisition is access to its massive cross-country fiber plant to connect businesses via dedicated WAN connections. For businesses with mission-critical voice needs, CenturyLink can deploy fully redundant SIP trunks onto these dedicated private WAN links directly into their network. Thus, customers can benefit from solid call reliability.

Negatives for CenturyLink SIP trunk services are similar to other legacy telecommunication companies, such as outdated contract options, poor self-service customer portals and inferior customer service compared to newer competitors, according to customer reviews from ConsumerAffairs and Trustpilot. That said, CenturyLink holds an excellent track record in terms of network reliability over the years. Current and new customers that are simply looking for a no-nonsense SIP service that can scale globally should consider CenturyLink.


Originally based in Grand Rapids, Mich., under the name VoEX, Inc., IntelePeer is a privately owned organization that has been around since 2003. Unlike traditional telcos that got their start with analog and digital phone lines, IntelePeer built the company on SIP alone. Thus, the company's network, expertise and marketing strategies fully cater to IP-based technologies.

Original IntelePeer customers were either large businesses or service providers that offloaded SIP traffic onto the IntelePeer network for cross-country transport. The company has since moved on to develop its own base of direct SIP trunking customers that range from just a few phones to tens of thousands.

While IntelePeer doesn't offer its own UCaaS platform, it does offer some hybrid services that can integrate with on-premises equipment over the SIP trunk. Additionally, the company has partnered with Cisco to be the back-end voice platform for the Webex Calling service. Thus, if you're looking for a company where you may eventually migrate away from on-premises UC to a cloud-based UCaaS platform, IntelePeer is one to keep in mind.

One of the negatives of IntelePeer compared to larger providers is that it is not a WAN service provider. The only way to connect to IntelePeer's SIP network is to build trunks over the internet. The company does not offer dedicated or direct connect services, so for businesses that require the utmost in reliability and call quality, IntelePeer may not be the right fit. Additionally, if a potential customer currently has telephone numbers it wishes to port over to IntelePeer, that process can be more complex and time-consuming. Lastly, IntelePeer has only recently begun to expand its services outside of the U.S., which can present a problem for businesses with a global need for SIP services.


San Francisco-based Twilio stands out among the other vendors here in what it can offer customers. The company touts itself as being a customer engagement service provider, as opposed to a more traditional communications service provider. Using a host of modern, cloud-based communications and analytics tools, Twilio customers can tap into next-gen applications to better reach customers. This includes cloud-deployed contact center platforms, programmable text messaging campaigns and advanced email APIs.

One popular Twilio product is Elastic SIP Trunking. This tool enables both small and large businesses to quickly deploy SIP trunks nearly anywhere in the world. The company offers a flexible, pay-as-you go model and on-demand scalability to expand or contract the number of simultaneous calls as necessary.

Formed in 2008, Twilio is publicly traded and has a current market cap of $13 billion. One thing to keep in mind, however, is that Twilio isn't a profitable company just yet. Thus, this may be a concern for organizations looking for a highly stable SIP trunk provider. While most analysts believe the company will soon turn a profit, it is something to keep in mind.

Twilio also lacks some traditional telecommunications service offerings that many of the larger competitors can deliver. This includes the ability to bundle internet and SIP trunk services into a single cost-saving contract. Additionally, while Twilio does offer dedicated SIP trunking connectivity, this service uses a third party for private WAN connectivity. Thus, troubleshooting becomes more complicated as more vendors are introduced into the equation. Twilio also suffers from the same expected issues as IntelePeer when having to port telephone numbers from one service provider to the next. Ultimately, customers should consider Twilio SIP trunks only if they also plan to use one or more of Twilio's other unique customer engagement services.


The second largest telecommunications company in the world -- and the largest in the U.S. -- is Verizon. With a market cap of nearly $253 billion, this company is considered one of the top three telecommunications leaders, along with AT&T and CenturyLink. That said, Verizon's primary revenue is found in wireless services, which accounts for nearly one quarter of its revenue stream. Thus, wireline services, including SIP trunking, are a much smaller part of the business. However, it's surprising to note that Verizon actually has a larger SIP trunk install base compared to AT&T -- and just behind CenturyLink.

The benefits of Verizon as your SIP trunk provider -- its service is known as IP Trunking -- are similar to AT&T and CenturyLink. Verizon is a large carrier with tremendous global reach. The company offers bundled telecommunications services together with SIP trunking services, including internet broadband and dedicated private WAN connectivity options. Additionally, if you're already a Verizon customer who connects your on-premises VoIP platform to the PSTN via legacy PSTN connectivity, porting your telephone numbers should be a simpler process.

With Verizon's customer service, some may find that contracts and pricing models are less flexible. However, for large businesses that require an extensive and reliable global service for distributed SIP trunk architectures, Verizon is a solid choice.


Privately owned, Colorado-based Voyant came onto the scene as one of the first pay-as-you-go SIP trunk providers just over a decade ago. Voyant offers a simple, pay-as-you-go model with no long-term commitments and a revolutionary self-service customer portal. While competitors have tried to emulate this model in the past few years, Voyant still remains a top choice for those that are looking for this level of flexibility. Current and prospective customers can also benefit from its 24/7 live customer support.

When comparing Voyant to other competitors on this list, it is most like IntelePeer in that Voyant largely bases its revenue and expertise on connecting businesses to the PSTN via SIP trunks. But, unlike IntelePeer, which partnered with Cisco to offer a UCaaS alternative to businesses, Voyant has its own cloud UC platform to offer. Voyant also partnered with Microsoft to sell a Microsoft Teams UC platform as an alternative. Thus, if your planning to migrate off on-premises/SIP trunk voice and video connectivity, Voyant provides more options to choose from.

There are some drawbacks with Voyant if you're not already a customer and want to keep your telephone numbers. Even though Voyant claims to have an easy number porting process, it's wise to assume there may be issues that can affect cutover dates. Additionally, Voyant offers no dedicated link connection to its data centers without having to bring in a third-party WAN provider. Lastly, Voyant's network resides only within the U.S. If you require a distributed SIP trunking architecture for offices located outside the country, it's more beneficial to choose one of the larger service providers that maintain global SIP services.

Editor's note

Using extensive research into SIP trunking providers, TechTarget editors focused on vendors that hold considerable market share and are comparable to each other in terms of financial strength, market share momentum, service development capabilities, and architecture and support options. Our research included Gartner, Trustpilot, ConsumerAffairs, Macrotrends and Yahoo Finance.

Dig Deeper on VoIP and IP telephony