Most manufacturers of IP PBX equipment (Aastra, Alcatel, Avaya...) have followed the same path, developing their own proprietary protocols.
The "stimulus" can implement easily any new facility telephone without having to modify the software embedded in the terminals, which simplifies the procedures for maintenance and upgrade of the installed base.
Nortel has been very active in the standardization effort of these protocols within the IETF, drawing on its work with pre-standard UNIStim and already developed work on the Nortel IP PBX systems and its IP Centrex platforms from 1996.
Contributions common between Nortel and Cisco Systems for example culminated in the publication of the IETF RFC 3054 "Media Gateway IP Phone Application Profile" outlining the options in the protocol Megaco/H.248 for IP command posts.
The UNIStim protocol is implemented on Avaya IP PBX systems and licensed by third-party suppliers → History See for example the release of the company Spectralink,[1] Details on the implementation of the protocol UNISTIM are available in the document "Telephony and Data Network Services at a Telephone", filed in the United States Patent No.