I cannot get a response from my modem when I try to give it an "AT" command

I cannot get a response from my modem when I try to give it an "AT" command

Explanation: The modem is not recognized, or is not responding. An attempt to communicate with the modem by bringing up communications software and issuing an "AT" command fails to elicit a response.

Diagnosis: Before you can even try to get a connection with your modem, you must first be able to establish communications to it, because your modem will not do anything if you cannot give it commands. Commands are issued using the "AT" command set. Inability to communicate with the modem is usually caused by configuration problems or resource conflicts with the modem or the serial port that it is using.

Recommendation:

The first thing you should check is your software setup. Make sure that your communications software is set to look for the modem on the correct COM port. Some programs under Windows 95 will automatically use the correct COM setting for the modem, but some may not. Under DOS or Windows 3.x it is likely that you will have to select the correct COM port manually.
Check all of the settings for the communications that you are trying to use. For example, you will probably need to select a speed (baud rate) for the modem, parity, stop bits, etc. Importantly, if you are using a non-standard COM port setup, meaning an IRQ that is different from the default one for the COM port, you may need to tell this to your communications software.
Make sure that you do not have a resource conflict. See this section for dealing with resource conflicts specifically on serial ports.
Make sure that you don't have two different devices trying to use the same COM port. This will definitely cause problems.
If your modem is an internal model and you just installed it in a PC that previously had no modem, there's a good chance that you have a conflict with your COM2 port. Many modems come preconfigured to use COM2 as a default. See here for ideas of what to do if you have a COM2 conflict.
If you have just installed an internal modem, or if you have just worked inside the PC on even an unrelated matter, you may have disturbed something inside the case or caused some other problem related to the inner workings of the PC. Try these general troubleshooting tips.
If you are using an external modem, check the power to the modem. Make sure that it is plugged in (check for lights). Try turning the modem off and back on again--sometimes they can get "hung up" and this will reset them to working condition. Check the transformer block to make sure it isn't overheating. Check the power cord.
Make sure that you have the right sort of cable to connect to your external modem. Ensure that it is plugged in securely.
For an internal modem, make sure that the modem is seated properly into its slot. Try reseating the card by removing it and reinserting it.
If the modem used to work and one day it does not, and nothing has changed recently, there is a possibility that it has been damaged due to a line surge after an electrical storm. This is unusual, but definitely not "rare". If you live anywhere that severe weather is common, this is far more likely. This can happen even if the rest of the PC remains functional, especially if you use power protection devices that protect the incoming power but don't protect the phone line.
If you are using a software modem or WinModem then part of the modem's functionality is implemented in software. The chances of a problem increase greatly when using this type of modem, which I do not recommend. See this section for more troubleshooting for software modems.
You may have a hardware problem with your modem. Double-check all the configuration items, and if you really cannot get the modem to work, try replacing it, preferably with an identical model. See if the second one works better.

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