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The error message might also be something like "Modem not responding". There are at least 4 possible reasons:

1. Your modem is a winmodem and no working driver has been installed for it. Or the modem is defective or in "online data mode" where it doesn't respond. See winmodem_
2. Your modem is disabled since both the BIOS and Linux failed to enable it. It has no IO address.
3. Your modem is enabled and has an IO address but it has no ttyS device number (like ttyS14) assigned to that address so the modem can't be used.
4. You modem does have a ttyS number assigned to it (like ttyS4) but you are using the wrong ttyS number (like ttyS2 instead of ttyS4). See wrong_ttySx and/or wvdial and/or minicom (test modem)

Case 1: Winmodem

For a winmodem with no driver (or a defective modem) the serial port that the modem is on can usually be found OK. But when the wvdial program (or whatever) interrogates that port, it gets no response since winmodems need a driver to do anything. So you see a message saying that no modem was found. However, it's likely that the modem card was detected at boot-time and it displays a message implying that a modem was found. So you're told both that the modem was found and that it wasn't found! What it all means is that no working modem has been found, since a modem that doesn't work has been found. Of course it could not be working for reasons other than being a winmodem (or linmodem) with no driver. See Software-based Modems (winmodems, linmodems).
Cases 2-3

Cases 2. and 3. mean that no serial port device (such as /dev/ttyS2) exists for the modem. If you suspect this, see Serial Port Can't be Found.
Case 4: Wrong ttySx number

If you are lucky, the problem is case 4. Then you just need to find which ttyS your modem is on.
wvdial

There's a program that looks for modems on commonly used serial ports called "wvdialconf". Just type "wvdialconf ". It will create the new file as a configuration file but you don't need this file unless you are going to use "wvdial" for dialing. See What is wvdialconf ? Unfortunately, if your modem is in "online data" mode, wvdialconf will report "No modem detected". See minicom (test modem)
minicom (test modem)

Another way try to find out if there's a modem on a certain port is to start "minicom" on the port (after first setting up minicom for that serial port. You will need to save the setup and then exit minicom and start it again. Then type "AT" and you should see "OK". If you don't, try typing ATQ0 V1 EI. If you still don't get OK (and likely don't even see the AT you typed) then there is likely no modem on the port. This may be due to either case 1. 2. or 3. above

If what you type is really getting thru to a modem, then the lack of response could be due to the modem being in "online data" mode where it can't accept any AT commands. You may have been using the modem and then abruptly disconnected (such as killing the process with signal 9). In that case your modem did not get reset to "command mode" where it can interact to AT commands. "Minicom" may display "You are already online. Hangup first." (For another meaning of this minicom message see You are already online! Hang up first.) Well, you are sort of online but you are may not be connected to anything over the phone line. Wvdial will report "modem not responding" for the same situation.

To fix this as a last resort you could reboot the computer. Another way to try to fix this is to send +++ to the modem to tell it to escape back to "command mode" from "online data mode". On both sides of the +++ sequence there must be about 1 second of delay (nothing sent during "guard time"). This may not work if another process is using the modem since the +++ sequence could wind up with other characters inserted in between them or after the +++ (during the guard time). Ironically, even if the modem line is idle, typing an unexpected +++ is likely to set off an exchange of control packets (that you never see) that will violate the required guard time so that the +++ doesn't do what you wanted. +++ is usually in the string that is named "hangup string" so if you command minicom (or the like) to hangup it might work. Another way to do this is to just exit minicom and then run minicom again.

Other problems which you might observe in minicom besides no response to AT are:

• It takes many seconds to get an expected truncated response (including only the cursor moving down one line). See Extremely Slow: Text appears on the screen slowly after long delays
• Some strange characters appear but they are not in response to AT. This likely means that your modem is still connected to something at the other end of the phone line which is sending some cryptic packets or the like.

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