Win 2000 incoming connections callback

We use win 2000 and have enabled incoming connections. With certain users the computer is set to callback. We use an USB smart modem and it will accept the users call and ask for their number to call then attempt to make the callback but will then stop and log a “Line Busy” error. This happens no matter who calls and there is nothing else on the telephone line conected to the modem. Can anyone help please?