[Users] Setting up voicemail

Hi all, I'm trying to setup an openser using an asterisk as a pstn gw and voicemail server. Everything seems to be working fine except when a call is redirected to voicemail by timeout (using failure_route). In this situation, asterisk is asking for proxy auth to openser that redirects the request to the calling ua. But when the ua responds to openser, this latest sends the invitation to the destination ua and not to asterisk, as he should. I can't figure out a way of accomplishing this... any idea? The sip trace diagram and the full openser.cfg are in attachment. The wrong invite is represented as "F19 INVITE" in the diagram... Thanks, Edgar