Execution     Response
Command       This parameter determines how the TA responds when circuit 108/2(DTR)
              is changed from the ON to the OFF condition during data mode.
AT&D[<value>]
              OK
              Parameter
              <value>       0     TA ignores status on DTR
                            1     ON->OFF on DTR: Change to Command mode with
                                  remaining the connected call
                            2    ON->OFF on DTR: Disconnect call, change to
                                 Command mode. During state DTR = OFF is
                                 auto-answer off.

When DTR is in high level, at the same time there is no on air or audio activity is required and no
hardware interrupt (such as GPIO interrupt or data on serial port), SIM300 will enter SLEEP mode
automatically. In this mode, SIM300 can still receive paging or SMS from network.
In SLEEP mode, the serial port is not accessible.
Note: For SIM300, it requests to set AT command “AT+CSCLK=1” to enable the sleep
mode; the default value is 0, that can’t make the module enter sleep mode, for more details
please refer to our AT command list.