Impact on registration of sip trunk

Is the purpose of SIP registration by the PBX only for inbound calls? (To notify the upstream provider where to send calls)

Or does it also affect outbound calls on a trunk? If a trunk registration fails, will it be ignore for outbound calls?

It is traditionally for receiving inbound calls. It may be a matter of policy (not of the specification) that it be used in some manner for outbound calls, for example for fraud purposes or identification purposes.

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