SIP trunking enables the end point’s PBX (Phone Exchange System) to send and receive calls via Internet. As SIP is applied for the signalling protocol for multiple real-time application, SIP trunk is able to control voice, video and messaging applications.  
It is also a voice over Internet Protocol (VoIP) technology and streaming media service based on the Session Initiation Protocol (SIP) by which Internet telephony service providers (ITSPs) deliver telephone services and unified communications to customers equipped with SIP-based IP PBX and unified communications facilities. Most unified communications applications provide voice, video, and other streaming media applications such as desktop sharing, web conferencing, and shared whiteboard.
The architecture of SIP trunking provides a partitioning of the unified communications network into two different domains of expertise:
- Private domain: refers to a part of the network connected to a PBX or unified communications server.
- Public domain: refers to the part of the network which allows access into the public switched telephone network (PSTN) or public land mobile network (PLMN).
The interconnection between the two domains must occur through a SIP trunk. The interconnection between the two domains, created by transport via the Internet Protocol (IP), involves setting specific rules and regulations as well as the ability to handle some services and protocols that fall under the name of SIP trunking.
The ITSP is responsible to the applicable regulatory authority regarding all the following law obligations of the public domain:
- Tracking traffic;
- Identification of users;
- Implementation of the lawful interception mechanisms.
The private domain instead, by nature, is not subject to particular constraints of law, and may be either the responsibility of the ITSP, the end user (enterprise), or of a third party who provides the voice services to the company
Each domain has elements that perform the characteristic features requested of that domain, in particular the result (as part of any front-end network to the customer) is logically divided into two levels:
- The control of access (Class 5 softswitch);
- Network-border elements that separate the Public Domain from the Private Domain, implementing all the appropriate ITSP phone security policies.
The private domain consists of three levels:
- corporate-border elements that separate the public domain from the private domain, implementing the appropriate company security policies
- central corporate switching node
- IP PBXs
- SIP Trunk guide 26 July 2018: SIP Trunks: A Guide for the Bewildered"'
- SIP trunking explained 26 July 2018: Making the move from PSTN to SIP trunk: SIP trunking explained"'
- "SIP trunking migration: Enterprise opportunities and challenges".
- "SIP Trunking Explained". Technology Convergence Group. Retrieved 8 September 2015.
- Gaboli, Ivan; Puglia, Virgilio (Jan 2011). "SIP Trunking the route to the new VoIP services". Kaleidoscope: Beyond the Internet? − Innovations for future networks and services, 2010 ITU-T, 13-15 Dec 2010. IEEE. ISBN 978-1-4244-8272-6.
- "SIP trunking explained". 2014-07-30.
- "Legal issues in different countries".
- "SIP trunking".
- "Role of Border Element". Cisco.
- "Acme Packet Net-Net session border controllers" (PDF). Acme Packet. Archived from the original (PDF) on 2011-07-17. Cite uses deprecated parameter
- "SIP Trunking Enterprise Solutions". Ingate Systems. Archived from the original on 2013-07-22. Cite uses deprecated parameter