The general syntax for issuing a modem command is:

@echo {AT_Modem_Command} > COMx:

where {AT_Modem_Command} is an valid modem command string and COMx: is the COM port that the modem is configured for.

Example - To issue an ATZ to my USR Courier on COM2:

@echo ATZ > COM2:

You can do this at a CMD prompt or in a batch file.