[Serusers] Need config help for PSTN call routing blocks to service provider.

Hi, I am new to SER and need experts' help in PSTN call routing block to call termination service provider in a scenario below: We have our own sip realm/server that will handle all VoIP calls among our own subscribers; sip.ours.com. This will forward all PSTN calls, authenticated and authorized by it, to sip.theirs.com, which will authorize usning our IP address and forward to pstngateway.theirs.com. What I have been given by SP is: sip.theirs.com, IP, myuserID and password. What they are given by me is our IP and hostaname. sip.ours.com and IP. My first try was simply; rewritehostport(sip.theirs.com:5060); t_relay(); ,which fails. Can someone tell me proper approach in ser commands based on the scenario above? Thank you so much in advance. John K