User-Agent

From Pbxnsip Wiki

In SIP, a "User-Agent" is a endpoint of communication. The name comes from the idea, that there is a software that acts as agent for the end user.

In many cases that idea has been taken much further. Technically, PSTN gateways are also User-Agents, but their "user" is the PSTN network. The B2BUA denotes the "Back to Back User-Agent", where two user agents talk to each other on their backs and look like regular user agents on the front. The pbxnsip is a B2BUA. This makes it possible, that on side of the PBX talks to an ITSP like a regular user agent; the ITSP might have the impression it is talking to a regular phone.