Anyone have any recommendations on pre-paid service/SIM cards for an unlocked "beater" phone? Wife and I both have smart-phones, where are great, but battery hogs. I picked up a cheap unlocked phone which has a week or so of battery life for backup, but I'm seeing so many choices for pre-paid SIM (GSM) cards, it's nuts. Does anyone have any recommendations for me? I'm looking for lowest-cost cards that work across North America without raping you on Canadian cost. Thanks!!