Modem Installed wrong?

My modem says it’s communicating but it doesn’t want to connect. The dialer connects ALMOST then, the error message opens 691. I have checked and rechecked the password and ID but that is not the problem. The modem used to work before I switched to cable, then when switching back the driver had to be downloaded. The computer store returned it without making sure it connects and although I know it should from previous use, they suggested it may be a bad modem?? The baud rate seems low (although this is just a guess..the phone cord length…ect.). I have tried to figure out what could be set wrong such as the phone, port, ect. and can’t quite get it myself. Thanks.