This is an interesting one. I'm running into issues with a fax application for a few numbers that are playing back a strange tone I have yet to identify. I originally thought it was FAS with international ringback, but it's occurring on a variety of numbers on various networks. The tone is playing immediately upon answer. Any help identifying what is causing this tone or what it is intended for is appreciated. The numbers are all fax lines and do get the proper CED after this tone. I can't find any matching tone in the T.30 ITU-T rec. As best I can figure, it's 400Hz with a 0.03s period on/off cycle, played 1s on 2s off. Listen at the below link. Sorry for the compression; email me and I can send you the original wav recording. https://clyp.it/vcq010th