How to wake up SIM800 module from sleep, after entering sleep mode 2 (AT+CSCLK=2)?
According to this page, to wake it up, you need to send any AT command, which will be ignored by the module (so no response), followed (within 5 seconds) by 


  AT+CSCLK=0


It took me all of one minute to find that by Googling "SIM800 sleep wake".