[Serusers] FAX problem behind NAT

I have an issue that I am hoping one of you has seen before. I have an ATA device connected behind a Linksys router doing NAT using the g711u codec. I am able to make calls without any problem and I get audio in both directions. However, when I attempt to use this line as a FAX, the FAX will not go through. Here is my dilema, when I move the ATA outside the firewall, the FAX goes through without a problem. I am not using T.38, I am just doing g711u pass-thru from an Azatel Sip Express ATA device to a Cisco AS5300. The fact that it works outside NAT but not inside leads me to believe it is something with my NAT config on SER, but since the voice calls work I don't know where to look. Any ideas or suggestions? I am stumped on this one. Thanks, Brian