What is SIP Calling? - A Comprehensive Guide

Written on Mar 12, 2024.

In an era dominated by digital connectivity, Session Initiation Protocol (SIP) calling has emerged as a pivotal technology, reshaping the landscape of communication. This article aims to demystify the intricacies of SIP calling, providing a comprehensive understanding of its functionality and the impact it has on modern communication.

What is SIP Calling

What is SIP Calling?
SIP is only involved in the signaling operations of a media communication session and is primarily used to set up and terminate voice or video calls. SIP can be used to establish two-party (unicast) or multiparty (multicast) sessions. It also allows modification of existing calls. The modification can involve changing addresses or ports, inviting more participants, and adding or deleting media streams. SIP has also found applications in messaging applications, such as instant messaging, and event subscription and notification.

SIP works in conjunction with several other protocols that specify the media format and coding and that carry the media once the call is set up. For call setup, the body of a SIP message contains a Session Description Protocol (SDP) data unit, which specifies the media format, codec and media communication protocol. Voice and video media streams are typically carried between the terminals using the Real-time Transport Protocol (RTP) or Secure Real-time Transport Protocol (SRTP).[3][8]

Every resource of a SIP network, such as user agents, call routers, and voicemail boxes, are identified by a Uniform Resource Identifier (URI). The syntax of the URI follows the general standard syntax also used in Web services and e-mail.[9] The URI scheme used for SIP is sip and a typical SIP URI has the form sip:username@domainname or sip:username@hostport, where domainname requires DNS SRV records to locate the servers for SIP domain while hostport can be an IP address or a fully qualified domain name of the host and port. If secure transmission is required, the scheme sips is used.[10][11]

SIP employs design elements similar to the HTTP request and response transaction model.[12] Each transaction consists of a client request that invokes a particular method or function on the server and at least one response. SIP reuses most of the header fields, encoding rules and status codes of HTTP, providing a readable text-based format.

SIP can be carried by several transport layer protocols including Transmission Control Protocol (TCP), User Datagram Protocol (UDP), and Stream Control Transmission Protocol (SCTP).[13][14] SIP clients typically use TCP or UDP on port numbers 5060 or 5061 for SIP traffic to servers and other endpoints. Port 5060 is commonly used for non-encrypted signaling traffic whereas port 5061 is typically used for traffic encrypted with Transport Layer Security (TLS).

SIP-based telephony networks often implement call processing features of Signaling System 7 (SS7), for which special SIP protocol extensions exist, although the two protocols themselves are very different. SS7 is a centralized protocol, characterized by a complex central network architecture and dumb endpoints (traditional telephone handsets). SIP is a client-server protocol of equipotent peers. SIP features are implemented in the communicating endpoints, while the traditional SS7 architecture is in use only between switching centers.

Security Considerations

While SIP calling offers numerous advantages, it's crucial to address security concerns. SIP-based systems can be vulnerable to various threats, including identity theft, eavesdropping, and denial-of-service attacks. Implementing encryption, strong authentication mechanisms, and regular security audits are essential to safeguarding SIP-based communication.


In conclusion, SIP calling stands as a testament to the transformative power of technology in communication. By embracing SIP, individuals and organizations can unlock a world of cost-effective, scalable, and feature-rich communication solutions. As the digital landscape continues to evolve, SIP calling remains at the forefront, driving innovation and connectivity in the modern era.