initiate dialing / connecting / sending SMS data on a digital connection, so serial ports, on the other hand, speak with a voice that is analog (if you do not convert analog to digital and send via digital form - VoIP), you should be able to connect to the speaker and a microphone. I would suggest using http://arduino.cc/en/Guide/ArduinoGSMShield → easy to use and support for all SMS (digital) and voice (analog). However, the application can use VB.net to get started.
hope this help.