SIP trunking
SIP trunking is 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 private branch exchange (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. Domains. The architecture of SIP trunking provides a partitioning of the unified communications network into two different domains of expertise: 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: 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 Architecture. 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 private domain consists of three levels: