What is SIP Trunking? Understanding with Real-life Scenarios

3 min readApr 26, 2022

What is SIP Trunking

SIP trunking refers to operating phone systems over the internet, in place of traditional phone lines. Let’s understand the terms SIP and Trunking one by one.

#1. SIP

An application layer protocol that enables a user to run a phone system over an internet connection instead of traditional phone lines.

#2. Trunking

The backbone of phone lines used by multiple users that connect to a telephone network.

With SIP trunking, a user (businesses/ individuals) can make and receive calls from multiple different clients or handsets, all of which share a dedicated internet communications channel the “trunk”. They also use the same protocol SIP to connect, maintain and disconnect calls.

#3. SIP Trunking use-cases

SIP Trunks facilitate Voice over Internet Protocol (VoIP) connectivity between an on-premise phone system and the public switched telephone network (PSTN). If your office has an existing PBX for phone service, SIP trunks facilitate phone service to the entire office so as to connect with the outside world.

SIP is capable of doing much more beyond basic telephony. Like traditional phone line subscriptions, SIP trunking is provided by a SIP provider. Most SIP trunking services allow a user to use their existing PBX equipment to upgrade to an internet-based phone network. SIP also lets a business synchronize all their communication channels in real-time, thereby enhancing their productivity, collaboration, and efficiency.

How does SIP Trunking work

SIP is a set of protocols or standards which describes how phone systems can:

Establish calls
Maintain the connection between phone systems for easy transfer of voice and data during an active call
Terminate connection when the call is over

The entire process involves phone systems sending signals to each other. These signals contain instructions to begin a call, play-back ringing sounds, stream audio or data between the systems, and terminate the connection. A call is established using SIP, from here other protocols handle the transfer of audio and data also known as media streaming between the phone systems.

SIP Trunks work as a link between your business phone systems and your ITSP (Internet Telephony Service Provider). Earlier enterprises had ISDN circuits physically installed on their premises. Voice calls were transferred over these circuits. SIP replaced this physical infrastructure with a virtual one, it sends data via networks instead of the copper wires of an ISDN.

Direct Inward Dialing (DID)

If you are an organization with a large headcount, consider installing a DID to effectively manage phone systems and save costs. DID enables a business to have multiple phone numbers. What’s more each of these numbers rings a specific SIP phone instead of going to a queue or an auto-attendant. One does not need separate phone lines for each number, the SIP provider sends an incoming call to your PBX over the SIP trunk.

The PBX then routes the call to the concerned recipient based on the number dialed. DID with a SIP trunk comes at no additional infrastructure costs, however getting DID services from a phone company involves additional costs for hardware and maintenance.

SIP Trunking vs PRI

Primary Rate Interface (PRI), uses a physical telephone line carrying multiple voice and data connections (for handsets and clients) to connect businesses to the telephony network. The difference between SIP and PRI:

SIP: It is an internet communications protocol. SIP trunks eliminate the need for hardware installations or services of telecom providers. It is much easier to set up, scale, and configure SIP over PRI systems.
PRI: It is a voice technology based on physical phone lines.

SIP Trunking vs VoIP

Continue reading: https://tragofone.com/what-is-sip-trunking/