What to check
The door ringing button has two screws on the back, check the wires are secure.
Short the two screws together to imitate the button being pressed.
If that works - Replace button.
Links
https://www.entryphone.co.uk/shop/productdetail/PB92-S
https://www.entryphone.co.uk/shop/productdetail/PB92-P
https://www.entryphone.co.uk/shop/productdetail/PB-VR
https://www.entryphone.co.uk/shop/productdetail/PB-VR-P
https://www.entryphone.co.uk/shop/productdetail/PB86