First and foremost, you require a provider with top notch coverage where you will be using the phone for hotline work. If any of your friends/coworkers in the area use cellular service and live nearby, I'd poll them first to see what their experiences with coverage is like, especially with the major carriers. Many of them resell server to the discount prepaid providers, so knowing ATT, Spring or Verizon's coverage will give you a good idea of which of the discount ones might be more affordable. Anyway, T-Mobile offers a startup deal of $100/1000 minutes which don't expires for 12 mos. This way you could choose when you topped off and which denomination you'd use to refill, using the 480min/mo x ~7 mos approximation as your top end/totals. Here's a link-
http://www.t-mobile.com/shop/plans/Cell-Phone-Plans.aspx?catgroup=prepaid-cell-phone-plan

I also heard Boost mobile is competing with a 10 cent/min rate right now. They resell Sprint service from what I recall. It really boils down to what the reception is for a particular provider inside you home/apt. In general I recommend the Nokia candybar style handsets, as they are really good at holding and placing calls in low signal areas. The Motorola RAZR has a decent reputation in this sense as well, although YMMV based on the quality of the network and amount of indoor signal you actually receive where you are using the handset. One thing I am not certain of is the trial period (if offered) when you sign up for prepaid. They give you a couple weeks usually with a post-paid plan to test the service where you live. This makes the information gathering you do before signing up more important if you don't get ample chance to test it out before your volunteering continues on your nickel ;). I can follow up with you more if needed, just do a little due diligence on your own if possible before you come back here. It would be helpful to know your zip code so I can check coverage for you as needed.

-Pedro