GSM command/function for determining wether incoming call is SMS or voice?

Hello,

I am having trouble with my GSM shield resolving how to determine what type of call is incoming, voice or SMS. I need to differentiate between the two so I can respond accordingly. I have look at the method below and I can't figure out if the incoming data is SMS or something else. I can't find a clear description on this, Any pointers that might help.

/**********************************************************
Method checks status of call(incoming or active) 
and makes authorization with specified SIM positions range

phone_number: a pointer where the tel. number string of current call will be placed
              so the space for the phone number string must be reserved - see example
first_authorized_pos: initial SIM phonebook position where the authorization process
                      starts
last_authorized_pos:  last SIM phonebook position where the authorization process
                      finishes

                      Note(important):
                      ================
                      In case first_authorized_pos=0 and also last_authorized_pos=0
                      the received incoming phone number is NOT authorized at all, so every
                      incoming is considered as authorized (CALL_INCOM_VOICE_NOT_AUTH is returned)

return: 
      CALL_NONE                   - no call activity
      CALL_INCOM_VOICE_AUTH       - incoming voice - authorized
      CALL_INCOM_VOICE_NOT_AUTH   - incoming voice - not authorized
      CALL_ACTIVE_VOICE           - active voice
      CALL_INCOM_DATA_AUTH        - incoming data call - authorized
      CALL_INCOM_DATA_NOT_AUTH    - incoming data call - not authorized  
      CALL_ACTIVE_DATA            - active data call
      CALL_NO_RESPONSE            - no response to the AT command 
      CALL_COMM_LINE_BUSY         - comm line is not free
**********************************************************/
byte CallGSM::CallStatusWithAuth(char *phone_number,
                             byte first_authorized_pos, byte last_authorized_pos)

Regards,

StanK