There are two kinds of "wireless" intercoms: Those that use the powerline for their communication, and those which use radio.
The powerline type sometimes don't work if they're not all on the same circuit. Some do work across circuits. But, the wiring in your house is actually divided between two "legs" of the incoming power. What comes into your house is three wires (four if you count the ground). Let's call the three "live" wires "A", "B," and "N." The voltage between A and N is 120VAC. The voltage between B and N is 120VAC. And the voltage difference between A and B is 240VAC. So, your dryer, your water heater, electric range, these sorts of "heavy duty" (usually heating) appliances get connected between A and B. But, standard plugs and lights get connected between either A and N or B and N. The idea is to wire the house so that about half of the plugs and lights are between A and N and the other half are between B and N.
A lot of wireless intercoms have problems communicating between one uhit plugged into an A/N plug and another unit plugged into a B/N plug.
For safety, electricians try not to put an outlet that's on the A/N side near an outlet on the B/N side. Very often, they do this by making all of the upstairs outlets be A/N and all of the downstairs outlets be B/N. One common use of an intercom system is to communicate between upstairs and downstairs.
The other kind of wireless intercom uses radio. The Reporter brand units are this way. They are essentially FMRS walkie-talkies without the walkie part. As such, they suffer the usual problems of wireless radio technology, interference, dead spots, range limitations, etc.
