Commercial systems are now available for the home with video capability, so you can see who is there and be able to speak with them as well. There are two types of intercom systems. The other type of intercom system uses carrier current. The intercom is integrated into electrical wiring, which carries the signal alongside the electricity. Components plug into outlets in order to transmit the signals.
Some static is picked up from the shared wiring.
These systems are generally less expensive than intercoms with their own wiring. Some systems are designed for ease of use, with disabilities in mind. When you shop for a system, learn the details of how it operates. Operation should be clear, intuitive, and not demand much memorizing of commands. Controls should be easy to operate without fine dexterity or the need to apply much force.