Most V.90 modems will attempt to make a K56flex (Kflex) connection before trying the V.90 protocol. USF modem pools support both K56flex and V.90 protocols. K56flex does not provide the same level of error correction that V.90 does. Disabling the K56flex protocol on a home modem is always a good idea. This can be done either by selecting V.90 as the primary protocol or by disabling the K56flex protocol.
The correct AT command to accomplish this can sometimes be found in the modem manual or on the modem vendors Web page. Most of the time a bit of trial-and-error is necessary to find the correct command. There are about five to eight different commands used by most modem vendors.
The easiest way to check for the correct command is using Hyperterminal (see the Troubleshooting section at the end of each operating system guide for steps on setting up Hyperterminal for this purpose). Try the commands until one returns an "OK", then put this command in the Extra Settings box under the modems properties (do not put the letters AT in the Extra Settings box)...
Here's some of the commands to try in Hyperterminal: