In using both ATA's and an FXS24 blade for hooking up a modem, I found max speed of connection was 33.6 at best(usually more like ~20kbps). Encrypted traffic over this connection would often fail. Specifically VPN connections would work, but with frequent disconnects.
Configure the ATA as if you were doing faxing:
http://www.cisco.com/warp/public/788/ata-fax.html
From Cisco docs:
"Success of fax transmissions up to 14.4 kbps depends on network conditions and fax modem/fax machine tolerance to those conditions. Network must have reasonably low network jitter, network delay, and packet loss rate."
