Another option is a prepaid calling card. Copy the card info (phone number, code, instructions) on a small slip of waterproof paper and perhaps laminate. I've been carrying the same card in my wallet for 7 years (bought it for use at the hospital when first kid was born, still not used it up). It has come in handy in situations where I don't have cell coverage (or battery charge), provided you can still find a pay phone. If some nice person lets you use their phone in an emergency you don't have to worry about making them handle the charges.