Here are some problems you might run into when you try to use a modem in Linux, along with some suggestions on what you can do about those problems.
Dead modem-For a PCI modem, check that the card is seated properly. For a serial modem, check that it is plugged into a serial port, that the phone line is connected (line, not phone port) and also that the electricity is plugged in. If one does not find the modem at all, it could indicate a hardware problem or an unsupported modem.
Not detected-If you have multiple modems, wvconfig might not have detected the right one (it stops after it finds the first modem). Disable the modem you don't want to use and rerun wvvdialconf.
Modem performs slowly-If there is a slow response when you type commands to the modem, there may be more than one device trying to use the same IRQ number. If you have an internal, nonworking modem and you attach an external modem, they both may be trying to use the same IRQ. To see what IRQs are assigned to each serial port, for example, you could type:
# setserial -g /dev/ttyS? /dev/ttyS0, UART: 16550A, Port: 0x03f8, IRQ:4 /dev/ttyS1, UART: 16550A, Port: 0x02f8, IRQ:3
This will query ttyS0, ttyS1, and so on to ttyS9 and display the device name, port type, hardware I/O port, and IRQ number for each. To solve the problem, you can disable or reassign the IRQ for the nonworking modem (or other device) that is trying to use your modem's IRQ.
Not logging in-Double-check the phone number, username, and password. Make sure there are no typos in your username and password. Watch the /var/log/messages file to see where the login fails.
Not getting a dial tone-Make sure your modem is connected to a telephone jack.
Modem keeps disconnecting-If you have calls waiting on your phone, incoming calls could cause your dial-up connection to drop.