Some older modems control this with a dipswitch setting, but most modern ones use AT commands to set it. To test this, use a terminal program to talk to your modem and issue whatever command your modem requires. The one remaining chore is to save this modem configuration change, so that it works this way every time you turn it on. To do that, get out the modem manual again and find the command that saves the current configuation as the default power-on setup. Make sure your modem configuration is really the way you want it, and then use your terminal program to issue the command to save the configuration.
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