Serial Port APIs
You'll need either javacomm20-win32.zip from http://www.idicalif.com/fsun.htm or RTXT from http://www.rxtx.org/
I used javacomm, but only because I was working away from home without an internet connection an that's all I had downloaded. I also wrapped the SerialPort class in my own one to enable unit testing without having a connection to the phone. This also has the advantage I can easily switch to rxtx at a later date if required.
You'll have the choice of polling the device, or using an event driven model. Disabling the serial events and polling is easier because you don't have to worry about events occuring while you're in the middle of reading a response.
Hint: Once installed our phone will be connected to a COM port. Check which one in Control Panel->Phone and Modem Options and viewing the "Modems" tab, then use HyperTerminal to test your connection (START->Accessories->Communications->HyperTerminal). When you connect with HyperTerminal you'll just get a blank (white) viewing area. Type ATE1 [return] to enable local echo, then AT&V [return] to test the connection futher.