> Has anyone worked on an interface to an internal modem using a pic?
>
> would it be possible to interface to the data bus(from the uart) and decode
> the address lines and use the irq for the interrupt?

I suppose it would be possible, but unless you add external latching,
you'll use up 15 I/O lines (8 data, 4 address, /IORD, /IOWT, and IRQ).  An
alternative might be to find a modem with a non-integrated UART chip.
Remove that, wire your PIC in its place, and you should be all set to go.