Using AT commands you're not restricted to a specific phone model, as long as the phone supports the commands you are using.
This program uses the text mode for sending the message, which is good for starting, but it may not be supported by every phone. If you want to use the PDU mode instead of the text mode - which is supported by all mobiles but is more complicated - see the PDU encoding sample for creating the required data.
Sample source code is available for VB6.
Details about the VB6 sample:
* Shows how to use the MSComm ActiveX control
* Shows how to use the SMS AT commands
* Includes a trace output to see what's going on
* Includes error handling for common errors
* Is not optimized for performance