[Users] sip trunking between carriers

Hello list, I have an installation with openser connected to a PSTN gateway and I am offering the possibility to my customers to rent a PSTN trunk to forward long distance calls to a remote SIP carrier via my proxy. My customer usually gets a sip normal account on from remote SIP carrier and my proxy must use to authentificate all invites relayed to this carrier. So my question is: how can I authentificate my proxy to the remote carrier with the credentials given by my customer ? Has anybody already implemented a similar solution ? I have checked the UAC module but it seems to give the ability to set only a static credential on FAILURE_ROUTE. Thanks, -vma .