I do have the wiring but still use battery operated type, main reason is I can buy three cheap Lidi battery types and have one in living room, one upstairs and one in kitchen all from same door push with ease.
Although the wired one did allow two sounders and two door bell pushes with different sound for front and back door it needs a lot of wiring.
Again although you can get transformers which fit inside the consumer unit, they have to be same make as consumer unit to maintain the type testing and have to be wired with 230 volt cable. Also they are normally AC only and limited voltages, so still likely will need an external transformer to match the door bell.
At mothers house it is hard wired to mains supply, 12 volt DC to the bell push/speaker/microphone and 13A wallmart to cordless telephone type charger which the cordless bell sits in, it can be extended to auto release the door at push of a button, however with all that technology my wife still opens window on stairs and shouts "I'll be down in a minute" so what was the point in fitting it?
We had a number of the plug into handy socket type, but one they take up a socket, two only one per door push, and three they never seemed to last long.
The three cheap Lidi door bells, clearly came with three door pushes, but set to all work off one, now around 8 years old and on second door push, still one spare.
The new breed door bell with answering system is a lot smaller and works with mobile phone not cordless, we tried getting ours to ring the cordless phone but it failed and even if you do get it to work, even with 4 cordless phones in the house (door bell classed as fifth) it will only work with one.
I have wondered if the new type which connect to mobile phones can connect to more than one?
With so many options I can see why not pre-wired, same with land line phones, years ago we hard wired land line phones, and fax machine, today fax is gone (it was 19th century) and we use cordless phones, I still have hard wired LAN but just two phones, set of cordless and emergency one which does not need mains power plugged in.