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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