The suggestion below is from Klaus Darilion

This should work as long as there is only one client registered. If there are multiple branches, step 2 must be done in the branch route.

#incoming call
# step 1
write src_ip $si to AVP
lookup("location");
# step 2
write destination IP $dd into AVP
compare AVPs
if AVP1!=AVP2 && natflag {use rtpproxy}

The following code snippet was presented by Tavis P.

else if ( isflagset(2) and isflagset(3) )
{
  log(1, "Both Clients are behind NAT");
  # Store the destination domain into an AVP
  avp_printf("i:450", "$dd");
  if ( avp_check("i:450", "eq/$src_ip/g") )
  {
    xlog("L_INFO", "Detected Two Clients Behind the Same NAT - Disabling Mediaproxy");
    # Do not use mediaproxy as the clients seem to be behind the same NAT
    resetflag(2);
    resetflag(3);
  }
}