Like a bunch of other people, I haven't been able to use my internal modem. 
(I have a G4 tower, the AGP version.)  I checked the list archives, so the
permissions for /dev/tty0[01] are set correctly (I think) and getty has been
turned off as well.  No reaction from cu or tip.  In fact they will
sometimes hog the console, so that I need to hit the restart button.
(Using ~^Z and kill is apparently not a good idea.)

I'm still using 1.5... would upgrading help?

Apple's overview of my machine says that the KeyLargo ASIC provides a
traditional serial port that interfaces with the modem.

http://www.netbsd.org/Documentation/Hardware/Machines/Apple/
then "Apple hardware developer documentation" gets you to
http://www.devworld.apple.com/techpubs/hardware/
then "Current hardware products" / "PowerMac G4"
then "Architecture" / "Block diagram and busses"
or "Architecture" / "KeyLargo I/O Controller"
or "Input and Output Devices" / "Internal modem"

Unfortunately there isn't much detail there.  I haven't determined yet how
much the kernel knows about KeyLargo.  Assuming we need documentation from
Apple, how do we get it?  I thought there was an ongoing process but I don't
know the details.

Thanks,

-- Derek