I once "rescued" an old manual set from an abandoned railroad line 'way out in the swamps. There was a battery box with it that held two 6 volt lantern batteries.
Bear in mind, current, rather than voltage is what makes phones happy, so if you can get around 20 milliamps at the far end, you'll be fine.
I'd say, get a couple of those lantern batteries, connect them either in series or parallel, and see what works.
jsaxe
