SIP and SIP Trunking



SIP (Session Initiation Protocol) is the underlying power of the Zultys MX IP PBX platform. SIP is an Internet Engineering Task Force (IETF) standard protocol for initiating an interactive user session that involves multimedia elements such as video, voice, and chat.

Zultys’ MX250 and MX30 are built on an open standards IP PBX platform based upon the SIP standard. Zultys systems provide a powerful platform to deploy, manage and use SIP based IP phones and other third party SIP devices. By supporting IP open standards, Zultys ensures that not only do Zultys IP Phones operate with the MX system, but a host of other devices, such as third party SIP-based IP phones, conference room accessories, paging equipment, application servers, video phones and video conference servers are supported, as well.

Zultys’ MX25 is a SIP gateway built on the same platform as the MX250. In addition to its gateway functions the MX25 can be used to expand the number of available PSTN module slots available in an MX250 or MX30. The MX25 may also be deployed as a stand-alone SIP gateway to IP-enable non-SIP compliant legacy telephone systems.

SIP Trunking

A SIP trunk is a service offered by an ITSP (Internet Telephony Service Provider) that connects a company’s PBX to the ordinary public switched telephone network (PSTN) via the Internet using the SIP Voice-over-IP (VoIP) standard.

Many enterprises want to make full use of their installed IP PBXs and not restrict themselves to communicating over IP only within the enterprise. A SIP trunk provided by an Internet Telephony Service Provider (ITSP) connecting the enterprise to the traditional PSTN network is the solution. Unlike traditional telephony, where bundles of physical wires were once delivered from the service provider’s central office to a business, a SIP trunk allows a company to replace these traditional fixed PSTN lines with PSTN connectivity via a SIP trunking service provider over the Internet.


Via the Internet Protocol, the ITSP provides connectivity to the PSTN (Public Switched Telephone Network) for communication with mobile and fixed phones.

By combining multiple ITSPs and managing them seamlessly within the Zultys MX platform, SIP trunks can offer significant reductions in operating expenses for enterprises, eliminating the need for local PSTN gateways, costly ISDN BRI (Basic Rate Interface) circuits, and PRI (Primary Rate Interface) digital trunking services.

Certified ITSPs

Zultys has tested and certified many ITSP providers for interoperability with the MX platform. Each provider supplies Zultys with a full connection which is used to perform an exhaustive battery of use case tests in order to certify all aspects of the ITSP’s functionality. A few of the elements tested and certified include:

  • Registration – Authorization
  • Call flow (inbound/outbound)
  • Call forwarding/call transfer (internal/external)
  • Call park and pickup
  • Conference calling
  • DTMF transmission reliability and recognition
  • Voice quality
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