So, it turns out the issue is still occuring. It definitely seems to be a disconnect supervision issue. If I call into the AA from the outside, dial an ext and hang up, then extension still rings even though the call has been "hung up".

Tested with a buttset at DEMARC, the line is taking about 5-6 seconds to disconnect.

Are the any settings in the SL1100 that I can play with to try to resolve this? Opening a ticket with the telco is likely going to result in multiple 'no trouble found' cases.