What is SIP Trunking? How to Record Calls in a SIP Trunk Environment

What is SIP Trunking? How to Record Calls in a SIP Trunk Environment
Last Updated on August 24, 2022

SIP Trunking is a revolutionary technology that allows businesses to take their telephone operations to the cloud. The technology is reliable and allows businesses to improve their communications in voice, video and instant messaging. Running a business phone system through a SIP Trunk is affordable and scalability is simple.

There are a lot of benefits to SIP Trunking and this article will discuss what is SIP Trunking, identify the pros and cons of the technology, and how to record calls on this system.

What is SIP Trunking?

SIP is an acronym for Session Initiation Protocol. A Session is a live communication such as a phone conversation, live video communication and more. Initiation means to begin. So SIP Protocol defines methods to establish and control media sessions (phone calls, video conferences or instant messages).

In other words, SIP is an application layer protocol that enables businesses to run their phone and communications through the internet as opposed to traditional telephone lines.

The term Trunking refers to the incoming line from the phone company or telecom provider. In a SIP Trunking scenario, the SIP trunk is a virtual connection between a company’s IP enabled PBX and communication services provider on the internet (cloud). Access to the Public Switched Telephone Network, or PSTN, is provided by the selected/subscribed communication services provider hosted on the internet. An internet connection is all that is needed to enable: phone calls, video calls, video conferencing and instant messaging.

Imagine a company with 40 external phone lines which was traditionally achieved by subscribing and wiring to several T1, E1 or ISDN PRI lines, or to a multitude of analog phone lines from the local PSTN lines provider. With a SIP trunk provider, all that is needed is an internet connection. SIP Trunking is becoming the industry standard and phasing out ISDN and other wired technology.

Note that VoIP enabled PBX, Internet Protocol private branch exchange or SIP compatible PBX, is recommended to transfer data. While VoIP/SIP compatible PBXs are recommended to successfully utilize VoIP/SIP Trunking, many users of the traditional PBX overcome this requirement with VoIP gateways. See here how to record calls from the PSTN or VoIP side of the gateway.

SIP Trunk Configuration:

SIP Trunk Configuration

Note: Other phones can be added before the PBX. Regular analog phones can benefit from the SIP trunk (PBX or Gateway converts traffic to VoIP).

SIP Trunks and SIP Lines

The power behind a SIP Trunking environment is the ability to scale SIP Channels, also referred as SIP Lines or Sessions. A SIP Channel refers to the ability of the SIP Trunk to establish a communication session/process between two points/devices/phones, in other words, the ability to support one incoming or outgoing call. For example, a SIP trunk with 10 channels can establish up to 10 simultaneous calls.

Businesses may scale up and obtain as many channels as needed for a specific SIP Trunk (limited by service provider capabilities or offering). This is a benefit for companies with large call centers because it allows one SIP Trunk to support hundreds of SIP channels. Multiple SIP trunks can be established if needed.

How to record calls in a SIP Trunk environment

VoIP Recording

There are many options to record calls in a SIP Trunk environment.

  • Recorder lines/channels can be tapped from the trunk side. A recorder NIC (network interface controller) is connected to a mirror port and decodes calls from the mirrored network traffic. For more details regarding this configuration, please see Scenario 3.
  • Internal phones/extensions can be recorded directly, see Scenario 4 and Scenario 5.
  • There are many other scenarios applicable to telecom systems with SIP trunks. For example, Scenario 9 employs a SIP trunk to send call data directly form the recorded phones to a call recorder.

SIP Trunking Pros and Cons

In SIP Trunk environments, there are both benefits and challenges to the system. This section identifies both the benefits and challenges of a SIP Trunk environment.


  1. SIP Channel Scalability – A SIP Trunk may support hundreds of SIP Lines. For businesses with a large call center, this provides an opportunity to scale and add multiple lines and enable simultaneous inbound and outbound calls. This allows businesses to only purchase the lines they need.
  2. Feature Control – SIP Trunks allow businesses to setup and control specific features on their system for customization such as SMS texting, sharing files and video conferencing.
  3. Higher Reliability – SIP Trunks allow businesses to install redundant phone systems. For example, businesses may forward calls to secondary phones if there is an internet service outage.
  4. Geographic Flexibility – SIP Trunks allow employees to use their SIP enabled device regardless of location. This solution is perfect for businesses with a remote staff. The only requirement is a reliable internet connection.


  1. Configuration and maintenance – Businesses that choose to have an on premise IP PBX system also must setup and maintain the network environment.
  2. Low Reliability – VOIP and SIP trunks are down when power, local network or internet connections are down. PSTN/TDM trunks are retained in some high availability sites due to this reason.

TDM Trunks Vs SIP Trunks Scalability

The rise of SIP Trunk technology is phasing out older trunk offerings such as analog PSTN trunks and TDM (Time-division multiplexing) trunks. TDM Trunks have been popular for many businesses for several decades. The technology relies on T1, E1 and ISDN PRI (Primary Rate Interface) and BRI (Basic Rate Interface).

One of the major benefits of VoIP/SIP Trunks have over TDM Trunks is the ability to scale easily by simply adding an additional line on demand.

When a company has an ISDN PRI trunk with a maximum of 23 lines and they need to add two-three extra lines, the business may need to:

  1. purchase a new PRI interface board and order extra PRI line service, which may have a requirement to pay for a full capacity of 23 channels regardless if all 23 lines are needed for business.
  2. or purchase an analog interface board and order extra analog PSTN trunks.

With a VoIP/Sip trunk, it may take only phone call and/or some software configuration changes to add those two-three extra lines.


Businesses are quickly discovering the benefits of SIP over TDM Trunks when comparing scalability, reliability, geographic flexibility and much more. As a result, VoIP and SIP based communication systems are becoming main stream.

Call monitoring is essential to improve call center performance for any business. This article has identified multiple VoIP/SIP Trunk environments with a call recording setup. Versadial call recording software solutions can be implemented in different configurations such as recording trunk (VoIP or TDM) side, recording extension side (TDM or VOIP extensions) or using a VoIP/SIP trunk to send data direct to a recorder. To improve the performance of your call center, contact us today to find a call recording solution for your business.

Previous Call Recording Articles:

PSTN Network Connection

PSTN vs VoIP – How to determine which technology is better for your business.

What are the advantages and disadvantages of PSTN and VoIP? This article explores both technologies and identifies the benefits and drawbacks of both.

How to record a phone call

How To Record A Phone Call

Trunk side or extension side call recording? What are the main things to consider and which side is better to record?

Common Call Recording Connection Using Mirror Port
Common Connection Using Mirror Port

How to record your VoIP calls

Do you have a hosted PBX or cloud VoIP provider? Learn how to take advantage of having your calls recorded and stored locally.

Last Updated on August 24, 2022