[Serusers] SER and inter-domain routing

Hello, This might be a trivial issue, but I can't think of a solution to it. I have a SER installed on a linux machine acting as proxy for our VoIP network I'd like to configure so that it takes PSTN calls and forwards them to the proper GW for termination. However this is a 3rd paty GW over which we have no control. I've setup the ser to use rewritehostport("host", "port"); in order to send the call to the PSTN-GW but the call fails every time with a 404 Not found This i reported back by the PSTN GW. I belive this is due to the fact that the <To> field contains our realm but the remote GW cannot process it. Any idea how I can get ser to work with this? Has any one setup something similar? Any help would be appreciated, Alex.