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SIP Trunk fundamentals: A guide to Session Initiation Protocol

IR Team

Written by IR Team


The world of technology is rife with acronyms – and Unified Communications (UC) has more than its fair share. Session Initiation Protocol (SIP) trunking is taking the business world by storm, and changing the way organizations communicate.

What exactly is SIP trunk?

Voice over IP (VoIP) phones are now the most common way for organizations to manage their communications network. SIP trunks are a replacement technology, and a modern alternative to the Integrated Services Digital Network (ISDN) systems that transmit data and voice digitally over physically wired connections.

Image Source: Business tech planet https://businesstechplanet.com/advantages-and-disadvantages-sip-trunking/

SIP trunks use an internet connection and Internet Protocol (IP) to allow businesses to set up real-time audio phone calls and/or video communication sessions between two-way or multi-party calls through Public Switched Telephone Network (PSTN). SIP trunking requires either an internet connection or VoIP vendor, an IP-enabled PBX and a gateway to support IP-based phone calls and video communication. 

How SIP trunks work

A trunk is just another name for a phone line. Before VoIP systems, each trunk had to have its own physical copper wire that connected that particular handset to the phone lines outside the building and the rest of the world. If you wanted to add a new phone, you needed to run a new wire, which meant cutting physical entry points in walls and floors.

With SIP trunking, all phone calls are transmitted over the internet instead, through an internet telephony service provider. This means you don’t have to pay the local phone company at all, instead simply adding your phone system to your existing internet subscription and paying for bandwidth accordingly.

What are the benefits of SIP trunk?

As many businesses have already phased out analog telephony, the benefits of deploying SIP trunk are growing. SIP is a lightweight, flexible technology consisting of a set of protocols delivered via a SIP trunk provider, allowing businesses to reap the many benefits of internet-based phones. Let’s look at some of these benefits.

Cost Savings

Until SIP trunking became more mainstream, businesses had to contend with the crippling deluge of charges associated with traditional Public Switched Telephone Network services, including local, long distance calls and international calling. Not to mention installation costs of hardware, maintenance and upgrades. One study revealed that the average business switching to SIP trunk saved 50% per month.

  • SIP trunking providers use the internet instead of expensive physical phone lines
  • Long distance and international calls become local calls
  • A SIP trunking service pricing plan is easy to understand, and is usually billed on a per-user basis.

Immediate return on investment

Many businesses are reluctant to plunge into new technology investments, due to the fact that they don’t see a return on their investment - often for years. SIP trunking technology comes with minimal investment costs and is a well-established method of communication, enabling businesses to see cost savings from their first monthly bill.

Global potential, mobility and flexibility

The fast scalability of SIP voice and data services allows businesses to scale with maximum flexibility. This means massive benefits for businesses looking to expand their organizations across multiple geographic locations and enable more full-time remote workers. SIP consolidates voice services and data into a single network, combining and connecting offices and remote workers. As business needs change, users and bandwidth can be easily added and modified without risk or unexpected costs.

Increased business productivity

SIP trunks are a gateway towards Unified Communications as a Service (UCaaS). SIP trunk providers can offer a full gamut of communication tools like video conferencing, instant messaging, collaboration apps and voice integrations, moving them to a single, manageable platform. With all your real-time communication handled from the one place, there is seamless connection between employees and offices – allowing for maximum efficiency and productivity.

Consolidating your network

Instead of paying for telephone voice services and data services separately, SIP trunking means that organizations can move their voice and data into a single network infrastructure, where voice calls become a form of data transmission. This can result in a significant saving, especially with fiber-optic internet connectivity.

Reduced need for physical infrastructure

With more and more businesses moving towards a hybrid working culture, physical office space and physically wired network infrastructures are becoming less of a norm. Every business knows the frustrations and costs of waiting for traditional telecommunications companies to install new phone lines. With SIP, businesses can add phone lines by simply connecting handsets to their data connection, Users working remotely can be easily integrated into the company network.

No additional costly hardware installation

With traditional telephony, referred to as Primary Rate Interface - or PRI phone lines, businesses often need to invest in costly additions to hardware as they scale their communications network. Increasing phone lines or add-on services is as simple as purchasing an additional handset and scaling SIP service agreements with SIP trunk providers


Nothing impacts business continuity more seriously than a breakdown in communication services. While traditional phone services are notorious for glitches, many people think the same applies to internet telephony. But SIP trunking provides backup in the event of power outages or system failures. Your SIP provider simply reroutes calls to employees’ mobile devices or other destinations. The failover capabilities of a SIP trunk service keeps businesses up and running, creating far more reliability than a traditional PRI phone system.

How many calls can be made at the same time with SIP trunk?

With PRI, or traditional telephony, a trunk referred to a bundle of physical copper phone lines which were part of a building’s physical infrastructure allowing multiple lines to be shared between users.

With modern SIP lines, this process becomes virtual. A trunk becomes a collection of assigned ‘channels’ which handle a set number of simultaneous calls. The number of channels you’ll require as a business is usually dependent on the size of your business, and how many concurrent calls you’ll likely be making and receiving at the same time.

Each channel allows one incoming and one outgoing call, but if your business receives a great volume of inbound concurrent calls, you’ll need a larger number of channels. Most medium to large businesses will benefit greatly from the flexibility and cost-effectiveness of SIP trunking.

What about bandwidth?

There are statistical models to help businesses know exactly how much capacity they will need at any time, considering peaks and troughs in call traffic. 

When determining bandwidth, a number of factors come into play, such as packet overhead, compression technology and network protocol in use. Too much bandwidth means overspend, which impacts ROI. Too little bandwidth results in blocked and abandoned calls which in turn harms a business’s reputation. Finding the right SIP provider to help calculate initial usage needs will mean less chance of running out of bandwidth, as well as significant cost savings. SIP trunk pricing varies depending on a business’s needs.

SIP – the new era of telecommunications

Technology is evolving at a rapid rate, but so too is our working culture. With remote and hybrid working now an accepted part of the way we do business, our communications infrastructure needs to be constantly updated. ISDN is now end-of-life technology.

Several countries worldwide have already made the switch to SIP, including the US, Australia, Belgium, France, Germany, Switzerland, Sweden – and the UK is set to switch off its ISDN in the very near future. 

This means that every business phone system currently using ISDN and traditional phone lines will need to migrate to SIP sooner rather than later.

Why upgrade from ISDN to SIP Trunking?

With SIP technology, there are no on-site components, so IT staff are free to manage other parts of the business. SIP service providers use a proxy server to provision all incoming, outgoing and PBX calls, as well as taking care of technical support.

SIP trunking means that service and power outages are no longer an issue, as SIP trunks automatically reroute calls.

The flexibility of SIP trunking allows businesses with an international presence or remote workforce to stay in constant communication with staff and customers across the country, or the globe.

The importance of monitoring your SIP trunk channels

As organizations continually shift with the times into modern communication and collaboration technologies, the management of complex, multi-vendor, hybrid communications environments becomes more challenging.

The success and seamless operation of your communications infrastructure depends on pro-active monitoring and performance management solutions. If you’ve already switched to SIP technology, or are in the process of migrating, it’s imperative that you have systems in place to perform detailed monitoring and troubleshooting tasks like:

  • Identifying the root cause of SIP call failures
  • Real-time VoIP traffic analysis
  • Correlating data on SIP trunk availability
  • Collecting call performance and corresponding network metrics
  • Alert generation

Find out more about SIP monitoring tools in our comprehensive guide here

IR Collaborate

IR’s solutions provide complete visibility across your entire UC ecosystem from a single pane of glass. Testing for, and identifying the root cause of voice traffic problems like  packet loss, latency and jitter is crucial to keeping your communications network up and running.

You need to be able to see what’s happening on your network that could impact VoIP traffic and QoS. For instance, whether backups are using up your bandwidth, or a router is misconfigured

Stress testing generates large loads into the system in a controlled fashion so you can confirm that the system is 100% connected to the telephone network and all SIP channels are available.

Whether your environment is on-premises, in the cloud or hybrid, IR’s solutions enable you to monitor, troubleshoot, and optimize complex UC environments with ease and deliver the best user experience possible.

Topics: Customer experience Performance management eBooks, Guides & Reports Collaborate

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