C1        C0 = turn off transmitter carrier.  You might do this
          to get the modem to be quiet so you could communicate
          by voice over a call.  It is unlikely you would never
          use this command.
          C1 = turn the transmitter carrier on so that it can
          communicate with the other modem.  This is the normal
          state.  This command is fairly rare.  Most modems do
          not support it.