REDIAL INPUT/OUTPUT ERROR

The first time after bootup that I dial in to my ISP, I get connected with no problem. If I disconnect and later try to reconnect, I get one of the following two error messages: i) Cannot open /dev/modem: No such device, OR ii) Cannot open /dev/modem: Input/output error. The same computer, modem and ISP work fine under Windows. Sometimes a solution is to log out and log in again; at other times I have to reboot. Has anyone had a similar problem? geno