That is because the Talkpath is designed to end recording by pressing any key. Once this occurs, then it will drop the call after just a few more seconds if no other options are chosen.
Since you already mentioned that the voice mail drops the call when you disconnect the active line, that pretty much confirms that the systems are set up properly and the disconnect signal is not being received.
Save yourself some time and try this test: Pick up on an available line and immediately place it on hold. If the hold condition drops off within about 30 seconds, then you are receiving the disconnect signal from at&t. If the line stays on hold indefinitely, they you aren't receiving it.
