SMS Modem?
Ok, you need to send AT commands to that modem.
AT commands originated with Hayes Microcomputer Products, and are a way of sending commands to a modem inline with other transmission data. The original set of commands has been extended over the years, and one area they cover is SMS
So in essence all you need to do is send data to the COM port that the modem is attached to.
And that's it.
Here's a list of AT command related to SMS:
SMS text mode Command Description
AT+CSMS Select message service
AT+CPMS Preferred message storage
AT+CMGF Message format
AT+CSCA Service centre address
AT+CSMP Set text mode parameters
AT+CSDH Show text mode parameters
AT+CSCB Select cell broadcast message types
AT+CSAS Save settings
AT+CRES Restore settings
AT+CNMI New message indications to TE
AT+CMGL List messages
AT+CMGR Read message
AT+CMGS Send message
AT+CMSS Send message from storage
AT+CMGW Write message to memory
AT+CMGD Delete message
SMS PDU ode Command Description
AT+CMGL List Messages
AT+CMGR Read message
AT+CMGS Send message
AT+CMGW Write message to memory