MOST look that way, but I've seen some pretty strange ones over the years.
And I've even seen doorbells hooked to the FURNACE CONTROL transformer (the one that operates the thermostat).
I've also seen a few older wired systems running off a lantern battery (12 volt) - a carry-over from the old 1.5 or 3 volt battery operated door buzzers of years gone by. ( using those big "ignition cells" ( the EN6 - now discontinued by Ever-ready))