I poked around on the US Robotics web site, and saw the following info on their Internet call modem.
"What is required to use the Online Call Notification feature of my Internet Call Modem? In order to use the Online Call Notification feature, make sure that you have Caller ID and Call Waiting provided by your phone company. "
Note that this only would allow you monitor calling numbers, and disconnect from the internet to take the voice call. These features are not really cheap, so I don't know if this helps.