[Users] UAC module use and REGISTER problem

Hi, I'm using UAC module in order to centralized authentication when I need to use an external proxy server to call PSTN numbers. To use it, I use a unique tuple login/pwd/domain The environment is: My openser handles the domain: "myopenserdomain.com" For each INVITE or REGISTER of SIP client of this domain, I force the authentication The external proxy handles the domain "externaldomain.com" Now I have two questions: 1) before to send an INVITE to the external proxy, must I change the contact header with the correct information of the tuple login/pwd/domain ? Or I must wait the 401 or 407 message before ? In fact I tests several external proxy. Some are waiting a contact header with the information of the tuple and some don't. What is the correct way to do this ? 2) Some of this external proxy accept only my INVITE (with or without contact header modified) if I registered first on it ! I receive 403 message in this situation. But I don't know how to REGISTER first since I don't find any openser module to that . How can I handle this situation ? Thanks, Regards, Christophe