What Are SIP Trunks and How Do they Work?

SIP trunking allows companies to use a telephone PBX for sending and receiving calls over the Internet instead of the standard Public Switched Telephone Network (PSTN). Installing SIP trunking modifies a Private Branch Exchange (PBX) box so it gains VoIP capabilities. However, SIP can handle more than just voice calls, including text messages, images, and video calls

What is SIP trunking?

Session Initiation Protocol (SIP) trunking is a service offered by a communications service provider that uses the protocol to provision voice over IP (VoIP) connectivity between an on-premises phone system and the public switched telephone network (PSTN).

SIP trunks running on PBX networks are basically virtual phone lines that enable users to make and receive phone calls over the internet to anyone in the world with a phone number. SIP trunks utilize a packet switch network, in which voice calls are broken down into digital packets and sent across a network to the final destination.

SIP trunks can host an unlimited number of channels – whatever your business has the bandwidth for. Fortunately, they are sold in small unit quantities such as two SIP trunk lines at a time, which makes the implementation very flexible in terms of installation and ongoing cost compared to traditional phone providers which sell bundles of 23 lines.

SIP trunking providers may offer additional services, including:

  • Encryption of voice calls to meet security needs;
  • Routing of calls to 911 services, with appropriate caller location information;
  • Failover to backup trunks and locations;
  • Ability to provision trunks via a web-based interface;
  • Interconnection with cellular networks so a call placed to a cellphone also rings on a desktop phone;
  • Protection against toll fraud;
  • Denial-of-service(DoS) protection
  • Virtual inbound phone numbers, including 800-number services;
  • Usage and performance reporting
  • Ability to route calls to multiple locations based on call volume, source of call or other policies.

What do I need to consider?

How many concurrent calls will you need?

Remember concurrent calls dictates the number of channels you need, and is important to note that this includes both inbound and outbound calls . It’s good to have a basic understanding of your business’s concurrency, but don’t worry if this is tricky for you. One huge benefit of SIP services is their flexibility – adding and removing channels is easy.

Are you porting any existing numbers over?

Unless you’re a brand-new startup, you’ll probably have a per-existing business number. Porting becomes essential as you don’t want to lose touch with your customers who are familiar with using a particular number for sales or support. Porting is simple but does take slightly longer than other elements of the SIP Trunk process, often lasting around 1-2 weeks.

How to Choose the Best SIP Trunking Provider?


A reputable partner will offer platform interoperability, meaning they can customize their services to your unique business communication needs instead of offering a cookie-cutter approach. Just as the most robust apps and other software are capable of being run on any operating system, a reliable SIP trunking system should be capable of operating in a wide variety of digital environments.

Data protection

A reputable SIP trunking provider will focus on protecting your customer’s data. Find out what security measures they have to protect against security issues such as toll fraud, malware, and various hacking methods in general. Be sure to inquire about their security protocols for not only your company, but their own as well.

Internet provider choice

Top suppliers will allow you to choose your own internet provider, which will make the SIP trunking services available in as many locations as possible. The major benefits of versatility and network cohesion are made possible through choice, and being able to use the internet service provider you already have a good relationship with is essential to ensuring your new SIP service conforms to your needs.


True SIP industry leaders should provide not only a low-cost option for migrating to new SIP trunking services, but also the ability to directly configure their service for operation with almost any brand of IP-PBX or softphone. If you’ve already set up a SIP trunking system in the past and found it functional but want to upgrade it, inquire about your new prospective SIP provider’s ability to use what is already there.


To ensure business continuity, ask about redundancy capabilities to seamlessly route calls in case of network failure. This is a key benefit of SIP trunking, with meaning that is sometimes lost on many SIP providers. SIP telephone networks can easily provide redundancy services that would be equivalent to having multiple hard-wired lines for a PSTN – and you’re missing out if you don’t ensure your SIP provider takes full advantage of this capability.


Verify if the provider you’re considering offers comprehensive training for SIP trunking and related technology. You and your stakeholders should expect no less, and expert training will go a long way to ensure that your new investment functions at the highest level right out of the gate.


Check on the level of automation your potential provider offers. Are their implementation, monitoring, and maintenance procedures remote and easy to manage? You’ll want to reduce the need for human intervention as much as possible while at the same time being confident that you or they can easily detect early signs that the system could be simplified.


Evaluate the SIP provider’s pricing model; is it straightforward and transparent? Do they require a lengthy contract and other restrictive terms? Are there monthly fees or upfront investment requirements? Top partners will offer prepaid and no-contract services with affordable monthly payments per unlimited SIP trunk channel. You should then be allowed to add, cancel, or upgrade channels at any time with a simple online control panel.

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