What is SIP?

Many people ask, what is SIP and why is it important? SIP refers to the Session Initiation Protocol which enables another type of protocol, Voice Over Internet Protocol (VoIP) to be initiated through making definitive and clear distinctions of two phases of a voice call and managing them accordingly.


What is SIP Communication?

SIP communication takes place as a result of two phases that are referred to as call setup and data transfer. SIP communication will initiate the call setup and connect two phones with one another, when this happens VoIP is triggered during the second phase where voice packets are moved between the two phones to allow a voice call conversation to take place. SIP communication is a supportive system to voice calls, video conferencing and even instant messaging. However, SIP is just one method for VoIP communications.

What is a SIP phone number?

A SIP phone number uses IP telephony in order to make and receive calls. IP telephony uses VoIP in order to establish connections and make phone calls. The process occurs through turning voices into data packets and sending them to the other devices.

The greatest benefit to SIP phone numbers is that they provide a very clear and direct connection between a private and public connection – very useful amongst telephone systems! This enables private businesses to call clients and members of the public without needing a legacy telephone system in order to be able to form a connection (1).

What is SIP Trunking?

Now that we have established what SIP is, we will explain what SIP trunks are all about. SIP trunking is a process that forms an interconnection between two domains and unifies them. This is accomplished through separating a network into public and private domains. Generally speaking, public domains are managed by an internet telephone service provider (ITSPs), whereas a private domain will be connected to a person’s personal server. In this case SIP trunks act as a bridge between the public and private domains to allow a connection to form. SIP trunks are essentially a collection of “channels” that are all assigned with a set number of calls that they are able to transfer simultaneously. SIP providers will help you calculate how many SIP trunks you will need in order to efficiently manage inbound and outbound calls. 

SIP trunks are particularly useful for businesses that receive a high volume of calls, they are a much more cost-effective and flexible method to establish connections, commonly used by call centres or support hubs.

History behind SIP

SIP was first discovered and designed by Mark Handley, Henning Schulzrinne, Jonathan Rosenberg and Eve Schooler in 1996. However, the protocol of SIP (referenced as RFC 2543) wasn’t officially standardised until 1999 (2).

The timeline of events that lead to discovery and implementation of SIP is as follows:

  • In 1875, Alexander Graham Bell made a successful phone call to Thomas Watson. It was very brief but gave the world a base to work from in order to create strong and successful connections.
  • Claude E. Shannon was a well known mathematician who provided the theory behind converting analogue to digital and then back again to analogue. Her theories provided the base for Internet Protocols and formation of VoIP.
  • In 1974, first phone call was made using an IP network discovered by Danny Cohen whom was using a Network Voice Protocol.
  • In 1995 VoIP was available to be used from home.
  • In 1996 SIP was discovered!

Credits to SIPTrunk for the timeline and history of SIP.

SIP plays a huge role in the telephony and computer industry and plays a key part in the growth of many businesses and how they use VoIP. SIP is a method that helps to simplify all forms of communication and even enables multiple connections to occur simultaneously. Without SIP trunks we wouldn’t be able to dial a 10 digit number and make a phone call, VoIP calls would only occur internally and traditional forms of communication would be required to make an outbound call.



The discovery of SIP has changed the way that we now communicate with each other. It has helped many industries perform their tasks more efficiently through e.g. conducting several calls at once no matter where the other line is in the world. For more information on how to implement SIP within your business, speak to our IT experts by calling on 0121 289 4477.

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