404 NOT FOUND/User not matched

I have a call scenario, a call is forwarded from a proxy router (e.g., SER) to
asterisk server. This calling user (user name in From Header) is not registered
in asterisk server but in other proxy server. The destination party is registered
in the asterisk server. The calling client received 404 Not Found and “User not
Found” error message is displayed in asterisk server. My question is how to
avoid the asterisk server check the calling user name. Thanks,