Looks like whoever installed this didn't understand basic concepts of using electricity in a reasonably safe manner. That much smaller wire spliced to the heavy wire would worry me, in the sense that the smaller wire has not enough over current protection. Also, it looks like those smaller wires are not going thru a raceway (that the right term, yes?) like conduit. Just thru a hole in the wall....
Unless you can get them to let you rewire everything in this place, I wouldn't do anything there. There's undoubtly many more serious mistakes around there.
I've seen both systems. Door phone systems are always 12V, but simple oldfashioned bells are available in 220V as well as in 12V versions. That one was a simple black 1950ies 220V bell. I rewired the entire circuit, but the old bell and pushbutton are still working.
There is a silver lining to this cloud. I was out at this building for the purpose of demoing out a lot of this junk for a new tenant. Although the EGC is run with the other conductors through the hole in the block wall, it's bonded to the box via a plastic masonry anchor. That 90° connector is a violation itself of 300.18(A). I'll send in a couple more pictures of this "pidgeon palace" to Bill...S