30miles is a long way to push battery. What would probably work if you can find them is 22 volt drycell batteries like telco uses. Put a battery on each end. On one end run the ring side of your pair to the battery, +or-, for sake of arguement say - put one lead of your butt set on + and the other lead to tip. On other end run tip side of pair to - of batt and butt set lead to + and to ring of pair. Should give you enough current to talk.

Retired phone dude