Handling inbound calls while dialed-up

[Michael O'Donnell]

Can somebody supply (or point to) an explanation of the
mechanics that make it possible for a dialed-up peecee
to receive notification of inbound calls?  I'm guessing
it's some trick based on forward-on-busy to a number
that somehow relays to particular port at my currently
assigned IP address, which would seem to require

 - some amount of coordination between telco and ISP.

 - some software on my machine prepared to respond
   on the port in question.

Did I also see that you can then digitize the calls
and have a conversation using the peecee's sound card
while dialed up?

Can Linux boxes can do anything like this?