I recently set up 8 Yealink T20P phones on 3CX Version 11. 3CX V11 is running on Windows 7Abyss. All the phones were registered manually without, as there was not an option for DHCP option 66. All the phones in the office register and work fine except one. At one extension, the user cannot press any buttons on his phone. He can receive incoming calls just fine and the system shows he is registered. I have reformated his phone, re-registered, upgraded the firmware (not to the latest, but at least to same one the others have--126.96.36.199), and still no luck. What would cause a phone not to be able to push any buttons? For example (he can't even dial out because when he presses a number, it does not appear on his screen and there is no tone). He can't change the volume either. Any thoughts?