What to check

CBP wire stands for 'Common Bell Push' and is the AC common feed to all ringing buttons.

If this becomes broken between buttons, some or all buttons will not ring.

Check the CBP wire connection and buttons for loose screws.

You should have 12v AC across each buttons 2 screws (assuming that there is a phone on the end of the ringing wire).

Corrective Action

Reconnect.

Additional notes

The CBP wire comes from the power supply or a relay board.