you could try what we call the parkway test set.. i belive the model number is pl-259. it is a tone generation set, a test set.. and it also has a talk circuit that runs off of 9vdc..

many splice and cable installation techs use this to communicate thru manholes while splicing.. you can run tone and talk at the same time..

you can attach a standard butt set (but it has to be a modern one with dual speaker phone setup) or you can use a headset with microphone..

not sure where you'll find one on the open market.. but they work great in the field for our techs..

depending on your wire guage you should be fine on the basic 9vdc..

the only thing you might want in addition is a separate circuit to run a buzzer as this system does not ring from one end to the other.. ..