C139 Tracfone
$14.99 Comes with 20mins prepaid. Cheap enough to find out if the service is right for you and your conditions. Get them and any Target, Wal-mart, etc...

I carry the C139 and work in a metal building. Excellent stand by time in that condition. Most cells drain themselves trying to keep in touch with the towers.

Or one of these other Tracfones.

PS. My total cost for the last year has been $14.99+99.99 for 400min and 1yr of service. About half of the iPhone cost.

For 14.99 + 139.99 You get the phone and 800min and 1 yr service Plus all future purchases of airtime come with double the mins.