I do this for contingency/backup reasons - Anotherlad, you can get a T-Mo prepaid (reg/micro) Sim for free or next to free when you get a Refill Card, depending on promotion in effect then. Most recently, a $30 purchase will get you both plus an iPhone compatible vehicle charger. It's easy to activate online with choice of most area codes & number, you can refill once every 90 days for as little as $10 and unused balance will rollover another 90 upon adding $10 or more to the account. The "cheapest" plan is pay-by-the-minute at 10 cents to 33 cents/minute, voice mail is included - text is 10 cents and pix/videos at 25 cents to send or receive.

No maintenance charges and you only pay/get deducted when the Sim in used - and you can switch to the $2 or $3 per day plan, and switch back as necessary. Info is a bit hard to find but here's the link -

http://prepaid-phones.t-mobile.com/prepaid-plans

Start the account with a $100 balance and you are automatically upgraded to "Gold" status, meaning that your unused minutes and balance are good for 1 year, and in year 2 - all you need to maintain the account is a $10 refill and it's good for another 365 days. Refill online by cc or by prepaid PINS from established online vendors - use mine for 6+ years and get another 10% discount so final cost is $9 every 365 days + usage.

Pickup a T-Mo microSim and get the micro-sim adapter and you can easily move it around around most GSM mobile devices (except for the iPhone5 with nano-Sim)