[Serusers] One path of RTP traffic

Hi, I have a SER + Mediaproxy. I have not any problem the call between SIP clients (behind or not the NATs) but when I try to call to PSTN (via cisco) I only have RTP traffic from SIP client to PSTN. Summarizing, the path of rtp traffic would have to be from: up: SIP Client ----> SER ---> GW-PSTN down: SIP Client <---- SER <--- GW-PSTN but, really is: up: SIP Client ----> SER ---> GW-PSTN down: SIP Client <------------- GW-PSTN I use the command: rewritehostport("212.xxx.xxx.xxx:5060"); when I match a geographic number. The complete scheme is: SIP Client ---- NAT --------- SER+Mediaproxy -------- SIP Server --- GWPSTN Some idea? Thanks, -- Alberto