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What is SIP Trunking?

SIP trunking (Session Initiation Protocol) is the virtual equivalent of a traditional business phone line. A SIP trunk is a virtual connection to the Public Switched Telephone Network (PSTN), utilizing your internet connection.

The term “trunking” dates back to the original days of telephone service. A trunk was a shared communication path for a large number of telephone users. SIP is one of the specific protocols that enable VoIP services to work.

While traditional phones use physical lines and circuits to connect to the PSTN, a SIP trunk replaces the physical connections with virtual connections over your existing IP networks. Because information is transmitted through the internet and not landline telephone lines, SIP integrates voice, video data and internet services into one single line.

With the invention of VoIP and PBX, voice communications were no longer restricted to traditional circuits and landline phones. SIP is a signaling language designed to carry data like voice, video and instant messaging. By using the internet, businesses can cut down on the cost and rigidity of older phone systems.

All SIP calls are designated as local calls and you won’t even need a phone line. Calls are connected via a unique address or SIP URI.

SIP Services are offered by most business VoIP providers for both small and large applications. They are a cost-effective way for small businesses to maintain a professional presence and for large companies to maintain a local presence.

How SIP Trunking Works

SIP trunking delivers your telephone and Unified Communications services over an existing IP network and replaces your traditional telephone lines. Through a SIP trunk, VoIP users can make calls directly to any phone on the PSTN without telephone lines by connecting to a compatible hosted PBX system.

The only three necessary elements to operate a SIP trunk are the SIP provider that supplies the services, a phone system that has a trunk side to enable SIP, and a border element to facilitate the connection between your enterprise network and the PSTN or external IP carrier network.

These border elements can be a firewall or switch and are usually managed by your SIP Trunking provider. With only these three simple elements any business can vastly boost its telecommunications reach. A pre-existing IP-PBX installed will only make the process even easier.

Advantages of SIP Trunking Services

Thanks to SIP, small businesses can easily appear large and professional with a level of service often reserved for more expensive solutions or Fortune 100 companies. Large businesses can maintain their local presence, as all SIP calls are local calls. Not having to dial a generic call center number is just one of the many benefits.


SIP’s inherent flexibility allows for users to add or even subtract as many phone lines as they need, without the hassle of overpaying for unused lines, or waiting for your phone company to sort it all out. Everything can be handled through dedicated web portals to tailor the phone system to your needs on the fly.

Prepared for Anything

While analog trunks can be disrupted by bad weather and rely on expensive phone companies, SIP trunking can help mitigate disaster scenarios by properly rerouting and prioritizing calls to different networks or phones.

Lower Maintenance Costs

Consolidated hardware and setups reduce the installation and maintenance costs of adding or changing your existing phone system. The system can grow without even adding new hardware, a new trunk for extra phone lines doesn’t require any new devices.

Keep Your Number

Take your phone number with you anywhere you travel or move offices, as your number isn’t bound by location. If your primary link to the SIP provider fails or just isn’t where you are, any internet connection can serve as a temporary link to your SIP network.

Call Anywhere

If your business does a lot of calling overseas, SIP can offer free international calls based on your provider and pricing plans (compare industry-leading SIP providers above).

How Can SIP Trunking Improve Business Communication?

SIP trunking replaces older connections that were more limited than IP networks. Adding phone lines to these systems required the purchase of a new trunk that included a minimum of 24 additional lines. If you needed just one additional line you had to purchase an expensive trunk with 23 unnecessary lines.

One of SIP Trunking’s strongest points is its scalability, it will grow or scale down as your business needs to.

SIP trunking combines video, data, and voice all into one single line. Your local, long-distance, broadband internet service, and video calls are all handled through one network. With dynamic bandwidth allocation, you ensure resources are available for basic web use while maintaining a consistent call volume and quality.

This gives your staff and your customers the best service possible.

With every service on the same network, the necessary hardware you need in-house drops dramatically. With SIP technology nothing is on-site, allowing your in-house IT staff to manage other parts of the business. Your provider will set up a SIP proxy server to provision incoming, outgoing, and PBX calls. They’ll cover your technical support to cut down on costs and headaches.

Service outages are a thing of the past with SIP trunking. If one location shuts down temporarily, SIP trunks automatically reroute calls to another location or extension to restore services. If there is an unexpected power outage, calls can be rerouted to a separate satellite office.

Companies with an international presence or a remote workforce love this benefit. The flexibility of SIP trunking allows you to spread staff across the country or the world.

Why Use SIP Trunking Over Alternative Solutions?

By sending all of your data over one single connection, it doesn’t take much to start saving money with a SIP trunk.

SIP trunks are over 25% cheaper than PRI trunks, lower calling rates aside. Beyond a level of flexibility unavailable in traditional phone services, SIP also allows you to make free voice calls between any offices on the same VoIP system. With a large list of providers and hardware taken care of on their end, pricing, installation costs, and rates for calls are all much lower.

Primary Rate Interface (PRI) trunking is another option that allows businesses to transmit a large number of voice and data communications from multiple locations. This is an older alternative to SIP. Since it relies on the use of T1 lines, telephone circuits might need to be added and calling fees are generally more expensive. Check out our guide to SIP vs PRI to learn more.

While PRI trunking can fail in bad weather, SIP services won’t let you down. They’ll keep you connected and reroutes calls during an outage or emergency.

As an internet-based replacement for traditional phone service, SIP is a central part of how VoIP works. It allows for a more robust and flexible unified communications system.

Choosing the Right SIP Trunk Solution

Once you’ve decided SIP trunking is the right method of connectivity for your business, you need to pick the SIP provider suited to your needs.


Some providers will rebrand a SIP trunking service hosted on another provider’s network. Make sure you distinguish if the network is hosted and run by your SIP provider directly. Everything is easier when you cut out the middleman.


Security in your SIP trunking solution is a reasonable concern for almost any service over the internet.

It’s important to ensure the necessary security measures are in place to keep your information safe. With all of your data transmitted over the Internet, your SIP trunk should be secured with a solid, protected network and anti-fraud system in place. This allows you to monitor any feedback and mitigate any risks to stay ahead of the curve.

Number Porting

If you want to keep your existing phone number, make sure your provider is capable of number porting from your specific region. Most services and providers offer porting. While there are limitations in different regions around the world, most providers have figured out a solution.

Customer Service

While SIP trunking is built to automatically deal with disaster mitigation, you need to have a backup plan. If everything goes down, you’ll want to make sure you can get ahold of your provider as soon as possible. Good customer service will save you from a five-minute outage that turns into a massive and costly headache.


As with any service, pricing will vary based on different providers and levels of service offered. Of course, you pay for what you get and if you want business-class service you should be prepared to pay for it. Those rebranding providers tend to be on the cheaper side but the quality might not be what you demand.

How to Get Started with a SIP Trunking Service

To properly set up a SIP trunk system, you first need to determine the resources that you will need.

  • Make sure you have a router and firewall setup with support for port address translations and static port mappings.
  • Get yourself a fully qualified domain name (FQDN) with a resolution to a static external IP.
  • Your bandwidth needs the capacity to handle real-time VoIP traffic, anywhere from 30-120 kb/sec.
  • Make sure that there’s a VoIP provider supported, of course.

After you have the appropriate setup, you can move forward into the actual installation.

Set Up an Account with a VoIP Provider

The first step to establishing a SIP trunk is to create an account with a SIP compatible business VoIP service provider.

Conduct a Test of the Firewall

The firewall between your system and the VoIP provider may not be able to route traffic if the configuration is not properly set up. As the middleman, test the firewall for compatibility for the SIP port and the audio port range. If the test does not work out, troubleshoot the firewall.

Add the VoIP Account

The next step is to configure the VoIP account into your system. You should see an option to add the SIP trunk settings in your user interface. Select the options appropriate to your geographic location and the numbers that are assigned to your new trunk.

Make a name for your VoIP account. Compare the details for your server hostname and outbound proxy with the details from your VoIP provider. Set the number of simultaneous calls that are allowed by your provider.

Set the authentication based on your registration details or your IP. Specify how the routing on your main number should be set up, and specify your DID numbers by adding the ones that are associated with your account. Set the number that you want to show up on your caller ID and don’t forget to save your settings.

Route Your Calls Over the New Trunk by Setting Outbound Rules

Now you can decide which kinds of calls will be routed over your SIP trunk. Create the outbound rules based on your needs and the capacity of your infrastructure.

Keep in mind that you can route calls through either SIP trunks or gateways. You can base these rules on the group or individual that is making the call, the length of the number or the number that person is dialing.

Make sure that the names for the rules that you set up are intuitive enough to translate between departments. Specify the elements that will trigger each rule.

For instance, you can create rules only for numbers that have a certain prefix, calls that come from a certain range of extensions, calls that go to numbers with a certain length or calls from a specialized extension group.

Specify how these outbound calls will be routed. Each new gateway and your SIP trunk will be listed in the route dialog.

You may also be able to set alternative outbound rules that will step in when the first set of routes is not available. Strip the digits and use the prepend option for each of the routes to transform the number matched with the outbound rule.

Setting up SIP trunking can create a much simpler communications structure within your organization and future-proof of your infrastructure for expansion into real-time enhancements. Expect some serious cost savings.

To keep your system up and running, follow our guide to avoiding VoIP hacks for SIP trunking systems.

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