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SIP Trunk fundamentals: A guide to Session Initiation Protocol

IR Media

Written by IR Media


The world of technology is rife with acronyms – and Unified Communications (UC) has more than its fair share. This guide is to explain the basic function and benefits of SIP trunk – or Session Initiation Protocol – and why it’s so important to your business and your communications infrastructure.

What exactly is SIP trunk?

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

SIP uses Internet Protocol (IP) to allow businesses to set up real-time audio and/or video communication sessions between two-way or multi-party calls through Public Switched Telephone Network (PSTN).

It requires either an internet connection or VoIP vendor, an IP-enabled PBX and a gateway to support IP-based voice communications. 

What are the benefits of SIP trunk?

IP telephony is the way of the future, as more and more businesses are phasing out analog telephony in favour of the benefits of deploying SIP trunk. SIP is a lightweight, flexible technology consisting of a set of protocols delivered via a SIP provider, allowing businesses to reap the benefits of internet-based phones. Let’s look at some of these benefits.

Cost Savings

All businesses are familiar with the crippling deluge of charges associated with traditional phone services, including local, long distance and international calling. Not to mention installation costs of hardware, maintenance and upgrades.

·         An SIP service uses the internet instead of expensive physical phone lines

·         Long distance and international calls become local calls

·         SIP trunk pricing plans are simple and easy to understand, and are usually billed on a per-user basis.

One study revealed that the average business switching to SIP trunk saved 50% per month.

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 has minimal investment costs and is a well-established technology, 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 a business’s needs change, it can easily add and modify users and bandwidth with risk or unexpected costs.

Increased business productivity

SIP is 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 productivity.

Consolidating your network

Instead of paying got telephone voice services and data services separately, SIP trunk means that organizations can move their voice and data into a single network, 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 remote 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 is now 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 calls you’ll likely be making and receiving concurrently.

Each channel allows one incoming and one outgoing call, but if your business receives a great volume of inbound calls at once, 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 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 businesses currently using ISDN  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

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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