I just realised I have the hardware (I think) for some serious call
handling testing: the 7170 fritz!box as TE connected to the ISDN
card with mISDN drivers in NT
mode. The 7170 has a built-in answering machine (either by default
or after upgrading to experimental firmware). Using the
auto-calling features of Asterisk I could just constantly setup
calls and either let the caller hang up (terminate call while the
answering machine still holds the line) or let the called party
hang up (let the answering machine hang up) and see what happens.
Originating calls on the other side (not asterisk) is also not too
complicated: just hook up a modem to the analog port of the fritz!box and
a few well-placed ATDT strings should do the trick.