SIP trunks can offer significant cost-savings for enterprises, eliminating the need for local PSTN gateways, costly ISDN BRIs (Basic Rate Interfaces) or PRIs (Primary Rate Interfaces). Why you would use a SIP trunk? Session Initiation Protocol (SIP) is used to initiate and manage Voice over IP (VoIP) communications sessions for basic telephone service and for additional real-time communication services, such as instant messaging, conferencing, presence detection, and multimedia. This section provides planning information for implementing SIP trunks, a type of SIP connection that extends beyond the boundary of your local network.
Deploying SIP trunking can be a big step toward simplifying your organization’s telecommunications and preparing for up-to-date enhancements to real-time communications. One of the primary advantages of SIP trunking is that you can consolidate your organization’s connections to the public switched telephone network (PSTN) at a central site, as opposed to its predecessor, time division multiplexing (TDM) trunking, which typically requires a separate trunk from each branch site. RFCs that discuss SIP trunking Best Practices for SIP Trunks:
Since SIP trunks are meant for interconnection between servers, they SHOULD run over TCP. Authentication SHOULD be done using mutual TLS authentication, with both sides of the trunk providing a TLS Certificate. TODO: might be interesting to recommend some practices for usage of phone numbers, but this might be out of scope here. Security Considerations: Servers providing SIP trunks will need to authenticate and authorize access to those trunk services. This specification recommends usage of the practices defined and required in RFC 3261 - mutual TLS uthentication - for this purpose. In some cases, the requests sent on SIP trunks can require confidentiality and message integrity. In such cases, usage of mutual authenticated TLS is RECOMMENDED. [RFC3261] Rosenberg, J. , Schulzrinne, H. , Camarillo, G. , Johnston, A. , Peterson, J. , Sparks, R. , Handley, M. , and E Schooler, "SIP: Session Initiation Protocol", RFC 3261, June 2002. [RFC3263] Rosenberg, J. and H. Schulzrinne, "Session Initiation Protocol (SIP): Locating SIP Servers", RFC 3263, June 2002.
Informative References: [RFC4458] Jennings, C. , Audet, F. , and J. Elwell, "Session Initiation Protocol (SIP) URIs for Applications such as Voicemail and Interactive Voice Response (IVR)", RFC 4458, April 2006. [RFC4480] Schulzrinne, H. , Gurbani, V. , Kyzivat, P. , and J. Rosenberg, "RPID: Rich Presence Extensions to the Presence Information Data Format (PIDF)", RFC 4480, July 2006. [RFC3903] Niemi, A. , "Session Initiation Protocol (SIP) Extension for Event State Publication", RFC 3903, October 2004.