I don't quite understand whether you are trying to use two modems. Certainly an asdl router will not be compatable with dial up.
The exclamation mark means there is a fault with the serial port. it will have to be uninstalled, the computer rebooted, and XP will instal drivers for the port.
However, check you have the latest drivers for the modem. but don't insal them until you see the serial port listed without exclamation mark