CMGS sends a reply to an SMS in memory. The argument is the index in memory of the SMS. To send to a phone number you want:
AT+CMGW="phone number"\rmessage(ESC)
That will reply with the number of the memory location where your messages is saved:
+GMGW: n
You save that 'n' and use the "AT+CMSS=n" command to send the message.