Hi,
This is not a NAP related question, but I believe I can help. It sounds like to need to disable call waiting. See http://www.modemsite.com/56K/callwait.asp
To do this, just add *70, (star seven zero comma) to the beginning of the phone number you use to dial your ISP. Another way of doing this is to go to the control panel and open the modem properties. There should be a check box there that allows you to disable call waiting.
Good luck
-Greg