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.