Does anyone know of a prepaid cellphone service (on a reliable network) where the minutes do not expire if you don't use them within a certain timeframe? I'd like to toss a prepaid phone using Verizon's network in my bag as a backup to my usual phone, but all of Verizon's prepaid plans expire if you don't use them pretty quick.

I'd really like to use Verizon's network because it is a stout one, but if you have good alternatives please post them.