I think that a prepaid card is a tremendous idea. It would allow you make a call whether it's local or long distance.

The magnetism from the compass may not be sufficient to damage the card. You could probably check with the phone company about that, or experiment with a card with just a minute or two left on it.

The only concern I'd have would be whether the card expires after a certain amount of time. It would not be a showstopper, though, you'd just need to be aware of that when storing the card.