One of our customers recently had a problem with an analogue version of the 2N Helios Force door entry unit that they had connected up to a Cisco UC540 system.
Calls were initiated with no problems but the system was not sending disconnect tones to the door entry unit, meaning that the call wasn’t ended until the button was pressed for a second time.
The solution ended up being quite simple! You need to change the analogue port settings in the UC540 to a managed user extension rather than a standard analogue port. The system should now disconnect the call properly when the call is hung up.