To answer IP calls means you need to implement one of the VOIP protocols
(e.g SIP or H323) on your answering side.
Also, VOIP call will work only if the source of that call is a VOIP URL.
To map the Phone (PSTN) calls to IP address, you need VOIP gateway
(e.g Cisco, Quintum, Sonus, Paraxip).
Or you can just terminate the phone calls to your computer using PCI Telephony cards like Intel / Dialogic.
But in either case, you will need some sofisticated software like VXML browser
that lets you hook the incoming call with database, file system, web apps etc.
[ January 13, 2006: Message edited by: Deep Narsay ]