asterisk users - Jul 2004 - who knows asterisk/libpri source code interaction

Hi,

Some ISDN Information has already been made available to asterisk eg.
{$CALLERID}. Since my C knowledge is very basic yet I'm trying hard to
find out how asterisk gets this Information from libpri.
I guess it could be found in chan_zap.c ??

Does anybody have more in deep knowledge of the asterisk and libpri
sources.

Any hint is apreciated

thnx

Moritz