I got fax working over pri to the 3cx fax extension. I had to enable Re-Invites and PBX delivers audio for the gateway in 3cx. But the real issue was under advanced configuration in the NBE, the value "netborder.media.t38.supported" was set to false. Set it to "true" and I was in business. I could see in the NBE logs that it was rejecting the 3CX Re-invite with: Code: 2011-05-22 20:09:26:357 -0700  INFO - netborder.voip.Sdp.summary : SDPMediaDesc0(Remote SDP T.38 offer is does not match local capacities, T.38 support might be disabled) 2011-05-22 20:09:26:357 -0700  INFO - netborder.voip.Sdp.summary : Remote SDP offer is invalid because not a single media description is supported. SDP offer : v=0 o=3cxPS 216912625664 417618460674 IN IP4 192.168.11.61 s=3cxPS Audio call c=IN IP4 192.168.11.61 t=0 0 m=image 10000 udptl t38 a=T38FaxVersion:0 a=T38MaxBitRate:14400 a=T38FaxRateManagement:transferredTCF a=T38FaxMaxBuffer:10500 a=T38FaxMaxDatagram:1500 a=T38FaxUdpEC:t38UDPRedundancy I cross checked all the other values and they were set correctly...the t.38 support was just disabled!