@sparkymarka Thanks for your reply - yes there's a video feed that appears on all the answerphones (which works). After the system timer would otherwise shut down the phones, however, they stay on, but not showing video -- just a blank (lit) screen. The audio is also cut.
The lock itself does...
I have moved into a (rental) house with a bpt 200 system, consisting of a VA/200 and several YC/200 answerphones, just like this: https://doorentrydirect.com/media/product_attach_files/1/x/1xmu_abk.pdf.
The system has one main problem, which is that it only provides 7V to the locking mechanism...