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SIP Trunking explained

SIP Trunking explained

For IT Managers looking to get ahead of the ISDN curve, SIP trunking is the ready replacement. Whilst terminology may be like your current PBX and IT environments, it is important to delve into what SIP trunking is, what it does and how it can help your business.

What is SIP trunking?

SIP is the protocol of VoIP communication that allows users to make and receive calls over the internet.

SIP works by sending messages from one SIP address to another. These messages are typically voice calls. However, SIP also powers messages in the form of video calling and instant messaging.

SIP is an over-the-internet exchange of information. A session is setup to initiate a transmission of these voice, video or instant messages. Once a SIP session is established, commonly referred to as a handshake, the data is sent, managed and ended by SIP. This sequence of events happens over milliseconds. This is how SIP calls can provide call quality better than that of a traditional phone line.

How SIP works

SIP initiates and terminates communications sessions, whether the session is a voice call person to person or a video conference between several parties.

The parameters of SIP only include the call itself. Equipment or software (endpoints) are required at either side of the call (SIP session). These could be standard telephone to telephone interactions, a blend of desk phone to softphone or from dedicated SIP devices like conference phones out to the external PSTN network where non-SIP users are able to receive calls as standard.

Once the session is initiated, speech or video is then transmitted as data over the internet and between endpoints. Once the conversation concludes, and the call is terminated by an endpoint, SIP terminates the session and data is no longer sent.

Network Impact

As SIP only looks after the calling aspect, a crucial part of the SIP journey is your network. For SIP to operate at optimal performance, your network must be ready to let it do so.

It is important to segregate a portion of your internet connectivity for voice. This allows the application of QoS to ensure the call quality is as best as it can be.

The amount of bandwidth you need to dedicate to SIP differs in every business. Working out how much is required is like planning ISDN trunks. Where you had 20 ISDN trunks, this meant you could have 10 people on the phone at any one time. If you still need 10 concurrent calls, 10 calls equate to 1MBps of bandwidth. 20 calls need 2MBps, 30 calls need 3MBps and so on. 

Lack of bandwidth, QoS or poor network configuration leads to degraded SIP call quality. Packet loss, jitter and latency have a negative impact on all parts of your network. As SIP calls power real-time conversations, the delay or quality loss is more obvious to the end user. Often, degraded network symptoms are displayed when calls drop mid call or audio is distorted and delayed.

Security within SIP trunks also relies on your network. SIP sessions themselves are secured by encryption of the SIP protocol for secure transmission.  Ensuring your underlying connectivity and network is secure end-to-end will allow SIP trunks to mirror this security.


Benefits of SIP trunking vary from business to business. We’ve included the most common benefits that most businesses see from day one.

  • Lower monthly rental charges – SIP and DDIs cost a fraction of typical ISDN lines.
  • Lower call charges – calls between sites become free, generally SIP rates are lower than PSTN and ISDN calling.
  • Flexibility – easily add channels to your SIP Trunk when you need to scale up or down
  • Capacity planning – no longer are you restricted by the amount of physical lines into your building, meaning you can correctly plan for call volumes.
  • Quality - SIP calling is superior to traditional on-premises, infrastructure based calling. The ability to apply and upgrade codec, adjust bandwidth and apply QoS ensures voice quality is as clear as real-time, always.
  • Easy administration - with a few clicks, you can setup business continuity plans, overflows for peak periods and personal preferences for DDIs.

Working with existing kit

When thinking about moving to SIP, it’s important to ensure your PBX is SIP-compatible. SIP is such a widely adopted technology that most PBXs in the modern workplace are interoperable. Be your existing PBX Panasonic, ShoreTel or Samsung, it’s likely that you can replace your current phone line infrastructure with SIP.

Other services like fax, card machines and alarms can also work over SIP. They may be programmed to work directly with SIP, or you may need an adapter to convert the SIP signals. 

Getting started

Now you know the basics of SIP trunking, you’ll want to dig further. Understanding how SIP can benefit your business and how it differs to ISDN technology of old are natural first steps. Follow our blog series to learn more about SIP and to see if it is right for your business.

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