Last Updated on February 18, 2026

There are several features in telephone communications available such as Caller ID, Call forwarding, Short Message Service (SMS), Local number portability, and many others. These features have not always been available. It was the creation of a protocol suite Signaling System 7 (SS7) which enabled these features and many others which telecommunications users utilize every day.

This article will define SS7, explain how it relates to the Public Switched Telephone Network (PSTN) and Voice over Internet Protocol (VoIP), identify the key functions in SS7, define SIGTRAN, and discuss how to record calls in an SS7 environment. This will be a high-level overview of SS7.

What is Signaling System 7 (SS7)?

Signaling System 7 (SS7) is a protocol suite or a collection of protocols that operate on different layers that serve different functions to provide end to end network connectivity. It is used to establish a virtual circuit across a telecommunications network and disconnect telephone calls on the PSTN. The protocol also performs number translation, call routing, prepaid billing, SMS, local number portability, call forwarding, caller ID, and other services.

How SS7 relates to PSTN

SS7 is often regarded as the backbone of PSTN signaling. It provides the signaling on the PSTN to set up, maintain, and tear down calls. SS7 also manages how calls are connected and maintained along different telephone exchanges around the world. It routes the call among multiple switching centers or central offices when a call is made.

What are the Key Functions of SS7?

1. Call Setup and Routing – SS7 protocols are used to establish a connection between the call originator and the receiver. It accomplishes this by determining the appropriate network resources, the correct route, and availability of the receiver phone. SS7 handles call rejection and/or the busy signal if the call cannot be completed.
2. Call Management – SS7 maintains the connection between the caller and the receiver when the call is established. This includes monitoring the flow of information, status of the call, and ensuring the call remains active. SS7 manages the tear down of the call when the call is complete.
3. Telecommunication Services – SS7 provides various services including:
• Caller ID – displays the number of the incoming call to the recipient.
• Call Forwarding – redirect calls to another number.
• Toll Routing – Routes calls to toll-free (800 and 888) and toll (900) numbers.
• Short Message Service (SMS) – Text messaging between devices.
• Local number portability – Allows users to keep their phone number when switching carriers.
4. Billing – SS7 is essential for telecommunications billing by tracking call duration, routing, and other call data information for accurate billing of local and long-distance calls.
5. Network Efficiency – SS7 utilizes Common Channel Signaling (CCS) where the signaling circuit is separate from the voice circuit. This contrasts with Channel-Associated Signaling (CAS) where the signal is delivered within the voice circuit. There is faster call setup time and reduced traffic to make more bandwidth available on the network in CCS due to the separation of the signaling traffic.
6. Improved Security – The signaling is carried within the voice path in a CAS environment. Both the caller and receiver have access to the voice path and may inject a signal and alter the signaling. The signaling is separate from the voice path in SS7 and CCS which improves security.

What is SIGTRAN (Signaling Transport)?

SIGTRAN (Signaling Transport) is a protocol suite designed to enable SS7 on IP networks to the PSTN. It was developed by the Internet Engineering Task Force (IETF) to map everything in the SS7 Suite right directly on top of the Internet protocol suite (TCP/IP). This enabled legacy telephony signaling protocols to function in newer packet-switched (IP-based) networks.

What was the need for SIGTRAN?

SS7 was designed for traditional Time Division Multiplexing (TDM) networks like PSTN. A method was needed to transport SS7 signaling over IP without rebuilding SS7 as telecommunications evolved towards VoIP. SIGTRAN was developed to enable existing SS7 applications to operate within an IP network. This allowed businesses to utilize the same key functions of SS7 (eg – SMS, number portability, etc.) and adopt the additional benefit of VoIP such as ease of scalability.

How to Record Calls in an SS7 environment

SS7 is utilized in analog and TDM lines on the Integrated Services Digital Network Primary Rate Interface Transmission System 1 (ISDN PRI T1). SIGTRAN was developed to enable SS7 on VoIP. Versadial’s call recording software is ideal for recording calls in both environments with several call recording scenarios.

Example: Call Recording Scenario #3

Several telephones are connected to the company’s PBX or phone system in this scenario. Environments utilizing SS7 via an analog or digital TDM (E1, T1, ISDN BRI or PRI) may have a VSLogger call recording server tap the line between the PBX and outside telephone trunk. Recording boards installed within the VSLogger server are required. VoIP environments utilizing SS7 via SIGTRAN may have a network switch with a Mirror (SPAN) port enabled. The VSLogger call recording server is connected to the Mirror port via a Recorder NIC. Calls in this environment will be passive, meaning the recording system will not actively participate. This means the participants will not be aware the call is recorded unless the company decides to communicate that information.

There are several other call recording scenarios available for both PSTN and VoIP with features such as active recording and others. Contact Us if you need help finding a call recording scenario that best fits your business needs.

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Last Updated on February 18, 2026