But my “outbound_auth/username” is 123456789 and it is correct and carrier confirmed this. I need also to provide authuser somewhere which is different. It was possible with sip but with pjsip it is not documented.
That is the username used for authentication in the authentication process. Are you actually wanting to register instead to a different AOR? You could also compare the REGISTER requests between chan_sip and chan_pjsip and based on that I may be able to answer more clearly.