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what is the cheapest method to make an intercom to communicate between ground floor ? Answered

How to make a local intercom to communicate between ground floor & 3rd floor in our apartment with actual telephones but without telephone line.  Is it possible?  If so please let me know the method in detail


We used to have a high-tech solution called ducting.  The furnace was on our ground floor and the ducting went all over the home.  Find a duct and yell into it and see if anyone can hear  you.

How about a speaking tube, like this, made out of a garden hose.

You can find intercom systems (or baby monitors with an intecom function) pretty cheaply, maybe even cheaper than making your own would be - randomly Googled example here.
I know it goes against the maker spirit to say so, but sometimes the COTS solution is perfectly fine, depending on your needs, resources and talents.

ONE method using 2 cordless phones is an instructable someone wrote using a simple 9 volt battery and a couple connectors...


Another method using this SAME 9volt battery and 2 connectors could be used on WIRED phones using your existing phone wiring... assuming you have phone lines in the locations you want to talk between.  The phone system only uses TWO of the four wires they wire your house with.  You will probably have 4 wires... red,green, yellow, and black.  The phone line only uses the red and green for most homes.  That leaves yellow and black for you to HACK and use!

Just two conventional telephones ? You don't want an exchange or other complications ?