[Serusers] Help...Setting rtpProxy and NAthelper behind nat

Hi folks! Ok, I got my rtpproxy working now, and the calls are comming thought it... but just if i have a real ip in my box.....but now I need to put my box behind a NAT.... I have an static ip mapped to an real ip in the nat router. . example: internal 10.1.90.100 --> outside 200.81.134.66 ...and now i need to call force_rtp_proxy but with the options to insert the outside address in the sdp and not the internal one.... I have tried several times with diferents values in the parameteres...but I can't get the result i want... i can not understand how i need to call force_rtp_proxy("f","200.81.134.66") i tried this but i didn't work.... Any help will be really appreciate! Thanks in advance! Armando Marrero Cti, Miami. FL