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Yes, it is possible, with a limitation. You can connect two phones in series or parallel to a 12 V DC power source (better use a resistor in series between the power source and the phones, a 220 ohm like the one I used should work). If you lift up both phones you should be able to talk, but you will not have any way to make the phone on the other end ring when you want to call.
Homemade Intercom (PBX) with GPIOView Instructable »