We ran this one down early in the game. Cisco never promises V.90 compatibility with the FXS boards. No change or fix for this was ever forthcoming. What we found was that some modems fared better than others - depending on which chip set they used - Rockwell or the USB. One or the other fares better - I forget which. But a little testing will show you. We also found out that cranking down the DB level on a port by port basis - Device, select FXS blade, click port in question, click the POTS icon and note Audio Signal Adjustment into and from IP network. Playing with these settings we saw better success at between -8 and -13.
In the end we got tired of monkeying with this and just put in real telco POTS lines for the few folks that actually use modems. (Most everything is IP based - what with our DS3 feeds to the Internet and all. Dialup is mostly archaic from a dial-out from the company perspective.) The FXS blades seem to do fine for FAX "analog" ports.
