Smart doorbells work with doorbell systems that use mechanical chimes and run on AC power; these systems are the most common type. Mechanical chimes have the classic 'ding-dong’ tone produced from a physical chiming mechanism. DC-powered mechanical chime systems, which are only present in about 1% of homes, are not compatible with most smart doorbells.
Slightly more common but certainly not wide-spread are doorbell systems with electronic chimes (also known as digital) that have a speaker and play a tune. These systems are often still compatible with smart doorbells if a diode is provided and installed at the chime. Check the manufacturer’s website to be sure that your existing doorbell system is compatible with the smart doorbell you are considering. Also keep in mind that any wired smart doorbell solution will have a minimum voltage requirement of at least 8 volts.
