You would use the VISA serial VI's since modems show up as Com ports on the computers. However, the trickier part of all of this is that your program will have to handle the modem communication parameters such as telling it when to take the phone off the hook, and what phone number to dial. Also, the receiving computer would need to monitor the modem and determine when the phone is ringing.
Look here in this
thread where someone else had a similar situation.
