Believe it or not, some people just don't yak on the phone. In fact, some people only have a phone in case of emergency. If you're one of those people, you're in luck. There are plenty of prepaid options out there that can fill your need without emptying your wallet.
What we're looking for here are providers that supply low-denomination cards that take 30 days or longer to expire. You're never going to find a cell provider that will let your $10 card last you forever, so we're looking for the best of what's available, not what's ideal overall. This also excludes providers that require minimum balances. These aren't ideal for this kind of situation.
We're really not discriminating against activation fees, since they are one-time fees, and you likely will be using this phone for a while. The cost of phone does come into play, if only because you don't need one of those glitzy phones that cost over $100. We're also going to discriminate against roaming charges; emergencies aren't going to happen at your convenience, and who knows where you might be at the time.
With that, onto the providers:

(
read the review) We're torn on Alltel. On one hand, the fact that their minutes never expire is highly attractive for emergency-only users. That their minutes are 15 cents per is even better. However, there is a charge of $4 per month if the phone is inactive during that cycle. And there's a $20 initial minimum balance.
The initial minimum balance we can live with; you're going to use the minutes anyway. It's just the $4 per month when inactive that gets us. We suppose that you could make one call per month, get billed the 15 cents, and go on from there. In fact, if you did that (you could even set up a reminder in your phone), this plan would be totally worth it.
Also working in Alltel's favor is its large coverage map. Yes, roaming calls are 59 cents per minute, but there aren't many instances where you'd be roaming. We can stomach the $35 activation fee -- consider it a "minutes never expire" fee.

(
read the review) AT&T has an incremental expiration date for prepaid minutes. That is, the more you buy, the longer it lasts. So if you're buying the smallest card -- $15 -- it will expire after 30 days. However, there is a deal hidden in here. The $100 card expires in one year.
So let's do the math. If you buy a $15 card, which expires in 30 days, you'd need 12 of them for a year, meaning you'd spend $180. However, you can just get the $100 card for the year. True, the per-minute rate may be expensive at 25 cents, but that means the $100 card gives you 400 minutes per year. Surely that will cover your emergencies.
Plus, you have the benefit of AT&T's nationwide network. Phones are cheap and there is no activation fee. Thumbs up to AT&T.

(
read the review) Boost is touted as a youth cell phone service, so we didn't expect much when we checked out their viability for emergency users. However, we were pleasantly surprised to find a 90-day expiration period. We can definitely deal recharging the phone every three months.
You can add as little as $15 to the card, so that means you'd be spending $60 per year. That would nab you 300 minutes. That sounds like a good deal to us. The only problem, as we see it, are the expensive phones. Then again, $40 for the phone means you're paying just $100 total for the first year. That works for us.

(
read the review) We'll be very honest: if you're only using your phone for an emergency, Cricket simply isn't the way to go. Don't get us wrong; we love the service to death and would get it were it available in our area. However, their plans are by-the-month, and they're all unlimited talk. If you're only using the phone for emergencies, we don't think you'll be needing unlimited talk. Still, if you need a phone for any other reason, this service comes with our recommendation.

(
read the review) There is nothing more frustrating than early minute expirations. Yeah, it looks great that Jump offers $10 airtime cards, but they expire in a week. We're not sure exactly what the point of that is: buying $10 airtime cards for a year means a $520 yearly phone bill.
Their most expensive airtime card, $50, expires after 90 days. Still, that's $200 minimum per year. Calls are cheap at 10 cents per minute, but we don't think you need 2,000 minutes per year. Add a $70 phone (the least expensive one) on top of it, and you have a service that is not recommended for emergency-only users.

(
read the review) While kajeet is aimed at a much younger audience (ages 8-14), it can be very useful for emergency-only prepaid users. The most attractive feature: money in your "wallet" never expires. You can add as little as $10 at a time, so that makes this a very cost-efficient service.
The only hitch, as we see it, is that there is a 35 cents per-day access fee, which works out to roughly $10 per month. Finding a good prepaid plan for emergency uses is a balancing act, though, and this seems like this is a good compromise.

(
read the review) LIberty Wireless only offers per-month plans, so they're probably not the best bet for emergency users. Using the minimum $30 per month plan, that works out to $360 per year. As you can see, there have been plenty of other services that offer better rates than that, so it's best to skip over Liberty Wireless for our current purposes.

(
read the review) Locus is another provider with incremental minute expirations, ranging from 30 to 90 days. However, unlike AT&T, in which the highest denomination card provides the best value, with Locus the lowest denomination card is most efficient. It costs just $10 and expires in 30 days; you get at least 66 minutes per month with that. So that means $120 for the year.
However, with the most expensive card, $100, expires in 90 days, meaning $400 for the year. And that just isn't going to fly. Still, the $10 cards make this service workable for emergency-only users.

(
read the review) You can basically look at our critique of Cricket to assess MetroPCS, since they both offer unlimited talk plans.

(
read the review) What we like about Mojo: you can get a $10 airtime card. What we don't like about Mojo: it expires after 7 days. Even the most expensive card, $75, expires after 30 days. Even with the smallest denomination card that expires in 30 days, $30, it would still run you $360 for the year. Sorry, Mojo, but that's a little much for our shallow pockets.

(
read the review) Unfortunately, Net10's earliest minute expiration just doesn't make it worth the service. We like the 10 cents per minute, but once again, that's not an enormous consideration here. What we're concerned about is that you have to spend $100 to get a card good for 180 days. That's $200 for the year right there. Once again, you can find better deals than this on minute expiration.

(
read the review) Well, here we go: a service that offers a $10 service card and a reasonable expiration period. That card gets you 71 minutes, and expires after 90 days. That means the minimum yearly fee is $40, which we're sure most people can deal with. The phones are really cheap, too, though they are greatly outdated. Then again, that shouldn't matter much for emergency users.
Will will add, though, that we've heard plenty of poor reviews of Omni's service. We only say this because the rates look so attractive for emergency-only users. Make sure to
read our review of this service, particularly the user reviews at the bottom.

(
read the review) Now here's a service we can warm up to. You can get a $10 airtime card that nets you 83 minutes. It expires after 120 days, too, which means you can get by on a little over $30 per year. Once again, this sounds like a sweetheart deal for emergency users, so you're best off
reading the user reviews before you sign up. The last thing you would want is a service that doesn't work when you need it most.

(
read the review) This one reminds us of the AT&T plan we described above. You can buy cards in smaller increments, but a $100 card expires only after a year. So that means you're limiting your costs to that, while having over 600 minutes at your disposal. To us, these are the best types of plans. Roaming charges are a bit steep at 50 cents per minute, but that should be manageable given the local rates.

(
read the review) We can't believe it, either: minutes do not expire with STi Mobile. So when you load those initial minutes on your card, they're yours until you use them. That means STi offers the most flexible emergency-only option of our reviews. You can get phones cheap -- or even free -- too, so that's another way to save with STi. The only downside: a 10 cents per day access fee. But that works out to roughly $3 per month, which is more than reasonable given this service.

(
read the review) Like AT&T and Page Plus, T-Mobile has a $100 airtime card that does not expire for one year. The lower denomination cards expires in 90 days, making the $100 card a far more economical option. You get a deal on minutes for buying that card, but once again, that's not a huge consideration. The phones start at $30, so you can get a decent deal on a functional phone. The large coverage map also helps T-Mobile's case in this instance.

(
read the review) Let us begin with this: we hate how it's so damn hard to find rates on the Tracfone website. You know, we like to know rates before we go ahead and make a purchase. Anyway, Tracfone actually has a plan for you: their 50 Minutes Value Plan. You are automatically debited $9.99 per month, and you get 50 minutes of airtime. We like this for emergencies, since it works out to $120 per year, which is about par for the course.
They also have cards that last for one year, available in 250 and 400 minutes. However, because of the website's setup, we weren't able to figure out exactly how much those cards cost. If they're under $100 -- and we suspect at least the 250 minutes one is -- that could also be worth it.

(
read the review) US Cellular only offers monthly plans, so they're not ideal for the emergency-only user.

(
read the review) The Verizon INpulse prepaid plans truly isn't a bad one. They have decent rates and offer a wide range of services when compared with other prepaid providers. However, their minute expiration makes them not very worth it. The lowest denomination card you can purchase is $15, and it expires in 30 days. So you're looking at $180 per year minimum. As we've seen, there are better deals out there. However, Verizon's reliability can play into this one. You may be shelling out $180 for the year, but Verizon's network is more likely to be there when you need it.

(
read the review) Virgin's minutes expires after 90 days, and you only have to add $20 each time. That adds up to roughly $80 over the course of a year, which makes it one of the better values we've seen. You can get a phone as cheap as $10, further adding value to using Virgin for your emergency purposes.
The only thing we'll add is that we've heard through plenty of first-hand sources that Virgin isn't all that hot for the above-20 crowd. And their customer service leaves much to be desired. But for the purposes of an emergency, we think their service holds up well.

(
read the review) Immediate disqualification: Xtreme Mobile charges a 60-cents per-day access fee, which works out to roughly $18 per month. That's $216 per year before you even make a call. So yeah, right out the window.