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If you're asking how to communicate with a modem, just open the COM port it's connected to and send AT command strings to it. If you have a USB modem it will show up as a COM device (look in the device manager for which port it is assigned to).
most of the modem used comm port, you just need to send the AT command character to the port, for dail number just sent ATD <number> <crlf> if you used the ascii hex the <crlf> is <0D0A> this char must put if not the modem will not accept the command