I haven’t seen one of the hard-wired units in many years. Wireless is the way to go. Mine sit on a counter or desk, but they could just as easily be mounted on a wall. Just plug in the power (or put in batteries), make sure the units are set to the same channel, and you’re good to go.
My parents’ house has an intercom system with two large speakers in the ceiling of every room. (The guy that built the house was an electrician.) We’ve never used it as an intercom system, but my dad did wire the XM radio to it so it can broadcast music through the whole house. It seems like he’s had to replace a lot of speakers and wiring, though, for a system that’s only about 20 years old.
The house we lived in before that had both an intercom system and a central vac. It was built in 1992. While you could theoretically use it to talk to whoever was at the front door, it was always too crackly and staticky to be of any use. If I ever built my own house, though, I’d defnitely install a central vac system. It’s perfect for anyone with dust allergies - no worries about HEPA filters or throwing dust into the air when you vacuum when the whole system vents to the garage or to the outside!
A security concious type who owned our house before us had an intercom system installed that was tied into the door bell. We didn’t bother with that except to aid sibling torment and even that was a novelty, we just felt a bit rude speaking to guests without seeing them.
The intercom worked throughout the house, but the layout of the house has the sitting and living room beside each other with the kitchen connecting them in a big U shape, so we’re really not far enough way to use the line connecting them. The bedroom intercom was handy if Mum or Dad were bedridden, been too long since I was at home to know if that still works.