[Serusers] RTP proxy/nathelper

As use of the RTPproxy consumes bandwidth on the SER/RTPproxy machines interface we would like to avoid it's use as much as possible. It appears that nat_uac_test detects our UA that are configed with port forwarding or DMZ as being behind NAT there by setting flag 6 and indicating the use of the proxy. As these do not actually need the use of the rtpproxy I want to force the use of the rtpproxy only when the calling party or the called party has a 5 in the 4 digit of the phone number eg 222522 or 123588 I have a clue on how to achive this when the called party has a 5 in the 4 digit by testing the uri but how would I test for the calling party. A code snippet would be highly appreciated. Regards, Dinesh