VoIP glossary · Back-to-Back User Agent

What is B2BUA?

A B2BUA (Back-to-Back User Agent) is a SIP element that terminates a call on one side and originates a new, separate call on the other, controlling both legs.

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B2BUA explained

A plain SIP proxy forwards messages and stays mostly out of the way. A B2BUA instead acts as the endpoint for each leg, so it can rewrite headers, change codecs, hide internal IP addresses, enforce call limits and generate accurate billing records. Most PBXs, session border controllers and softswitches behave as B2BUAs.

Because a B2BUA owns both legs, it is also where many interoperability problems get fixed or created. If a call works between two systems directly but fails through a B2BUA, compare the INVITE on each side to see what was changed.

Example

Asterisk receives an INVITE from a desk phone, answers that leg itself, then sends a fresh INVITE with its own Call-ID to the SIP trunk. The phone and the carrier never talk to each other directly.

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Put it into practice

Prepaid SIP trunks and wholesale routes from $200. Ask sales for rates or the NOC for a technical answer.

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