The same principle is applied when you use call forwarding on most SIP providers. They "spoof" the real caller's Caller ID when forwarding the call, so that the forwarded call reaches your phone with the number of the actual caller, not some random number assigned to your account or the provider. If they didn't do this you wouldn't be able to call the original caller back directly from your call log.

This would be a legitimate use case for Caller ID spoofing.