[Serusers] One way audio

Can someone help? I am fairly new to SER and Mediaproxy and can't find a detailed explanation of how nathelper etc work, so I'm struggling! My config is:- SER 0.9.6 Mediaproxy 1.7.2 Fedora Core 4 86x64 SER and Mediaproxy run on the same server which acts as a gateway to a remote Sonus PSTN gateway over a VPN. I also use Asterisk on my local network. The SER server acts as a router between Asterisk servers and the PSTN via the Sonus. This works fine but my problem is when I attempt to use X-Lite 3.0 UA behind NAT on a remote network. Asterisk is not used in this config but proves that two way audio is fine using SER and the Sonus gateway. Problem: X-Lite behind NAT on remote network to Sonus PSTN gateway via SER and Mediaproxy, I get only one ay audio. The audio is fine from X-Lite to PSTN but not in the reverse direction. Thanks for your help