[Users] CID insertion

I have my user authentication in a MYSQL database and use openser administrator to manage users. With every user I can add the Caller ID which ends up in the database (I've confirmed the existence in the DB). I'd like to add a Remote Party ID header into my calls directed to my PSTN termination provider (this is the header they want). I have had a look at the Auth module, and if I understood correctly, needed to add the lines modparam("auth", "rpid_avp", "i:13") to obtain the rpid field from the database and append_rpid_hf(); to actually add the RPID field to my outgoing INVITES, etc. however, I still can't see the header being added to the calls in outgoing SIP traces, and I have the feeling that I'm missing something fundamental.. I've tried also adding this line to load the credentials from the database: modparam("auth_db", "load_credentials", "rpid") Do I need to set the load credentials to be i:13=rpid to load the rpid field into the avp??? It's not 100% clear from the documentation where and how these avps are used (automatically if they're filled, or if they need to be somehow referenced in the function that uses them) and exactly which function is necessary (do I need the auth_db function or not?)..