I've heard of people getting a yagi antenna for the frequency and putting it on their roof pointed at the nearest cell tower. They have to connect their phone to the cable from the antenna to make a call and most phones don't have an antenna connector. I don't know if a passive repeater would work and an active cell repeater is very expensive.
If you've got an old satellite dish (or even a metal trash-can lid) you could try aiming that at the cell tower and positioning the phone at the dish focal point.