What is the cheapest wireless audio intercom?

Question

Do it yourself

The office requires the simplest intercom so that people can call the secretary directly from the street. The landlord forbids laying the cable, but the wireless was agreed. There are many different video intercoms on the market, but the video functionality is redundant, and you don’t want to overpay for it. The outdoor unit must be protected from rain.
Maybe someone has met inexpensive audio models of wireless intercoms?

Answer 1

buy a cheap walkie-talkie and insert what you have into the wall \ door, bring out the button and bring power. I saw rubles for 500 Chinese

Answer 2

You can just have a nice sign with a phone number :). A baby monitor is also an option.