The USB modem has default software with the driver for PC to make calls and to receive calls which work fine. I want to program it myself if possible with Telephony API so that I have more control and I can process the voice data.
Moreover, there is sample application in codeproject(http://www.codeproject.com/Articles/10994/TAPI-3-0-Application-development-using-C-NET) I tried too. However, the code gave an error saying a dll is too old. Changing this line,
bcc = ln.CreateCall(textBox1.Text,TapiConstants.LINEADDRESSTYPE_PHONENUMBER|TapiConstants.LINEADDRESSTYPE_IPADDRESS,TapiConstants.TAPIMEDIATYPE_DATAMODEM|TapiConstants.TAPIMEDIATYPE_AUDIO);
to this,
bcc = ln.CreateCall(textBox1.Text, TapiConstants.LINEADDRESSTYPE_PHONENUMBER, TapiConstants.TAPIMEDIATYPE_AUDIO);
I had been able to compile and run the application. Whenever, I select the modem line and press call providing a number it gets disconnected just after dialing. Hence, I have misgivings whether this TAPI application had a different purpose and may not work with USB GSM Modems.