I'm working on a small PBX install and I'm having trouble with two of four CO lines which are not disconnecting when the caller hangs up. In my conversations with the Verizon CSRs the only thing that seems to be different with the service ordered on the lines is that the two lines which don't disconnect cleanly are set to hunt to other lines on a busy. Any clues on how I might go about resolving this - I'm doing this job as a semi-knowledgeable amateur for a non-profit organization, so there's not a lot of budget to play with.

Details:
Four POTS lines - A B C D

A, B and C have caller ID and hunting A -> B -> C

D was provisioned solely for the fire and burglar alarm systems and has no enabled "features"

When testing each line individually connected to the PBX, A and B do not seem to be disconnecting quickly when the caller hangs up, but C and D do.

I've worked around the immediate issue by reprogramming my auto-attendant to disconnect on no entry timeout. However, all would be neater and cleaner if the lines would signal properly so that the PBX could know to hang-up and send a disconnect signal to the AA.