What to check
Door release is normally supplied with 12v AC only when the lock button is pressed on any phone.
With double doors, check the door loop or hinge contacts if there are any fitted.
Corrective Action
Reconnect or replace door loop.
Links
https://www.entryphone.co.uk/shop/productdetail/DL-B
https://www.entryphone.co.uk/shop/productdetail/DL-ARM