EDIT: I just saw that this is essentially the same as Door station keeps ringing
We connected the device to our hosted Asterisk-based system (we haven’t tried the office UCM yet). Set the number for the door button to be a SIP extension (phone) and pressed the button. Desktop phone rings almost immediately.
If the call is picked up within the first second or so, then normal two-way traffic is generated, but within 5 seconds the door chime plays on both sides. VERY LOUDLY.
If you wait 3 seconds or so before answering, this does not occur.