We bought an intercom system from Radio Shack back in the late 50's? (maybe early 60's) that just plugged in to an outlet. As long as both units were on the same side of the electrical panel, it worked great.
Mom could talk to whoever was in the shop if there was anything important - like announcing that lunch was ready.
Don't know if anyone makes those anymore; if they do you could buy the number of stations that you need.