Function:   IsPort()

               this function checks if the COM port is present, then,
               assuming that a Modem (and a phone) are hooked to it, you
               could use it for dialing for Voice.

   Syntax:     IsPort([<nPort>]) --> lPortPresent

   Arguments:  <nPort> in the range 0..3 indicating COM1 Though COM4.
               When none supplied, isport() looks for COM1.

   Returns:    .t. if a COM port is attached, .f. when there nothing.
               You can only use AtModem() and Dtr() when IsPort() returns
               True.