[Serusers] Simple proxy

Ok, I have looked at all of the suggestions made on this list for setting up a simple SIP proxy, but I am still confused. Perhaps if I fully explain what I am trying to do, then maybe someone could point me in the right direction. I am trying to use a proxy to help to secure a Cisco PSTN gateway. I currently have the Cisco setup to block all voice traffic, except for certain IP's. This works fine for most cases, but we a have a customer that has a server that "releases" the call, so I have no idea what IP's their phones are coming from. This causes their calls to be blocked. I hope someone out there has some ideas on where to point me, or has experienced this before. Thanks, B. J.