Cell phone companies are often highly motivated to maintain a very high percentage of service. Last cell site, one of their hubs, I worked on was making something on the order of $150,000 an hour in charges.

It was also highly redundant. Four separate fiber lines, 20 pair copper(mostly used for control and reporting) and a buttload (the tech's term) of microwave channels. It had two backup generators each supplied with better than three days propane. It was also the site where the company stored their generator trailers so there were always another two or three ready to go.

All that said we were called into correct an issue with their power transfer system. Power went out and failed to start the sequence of transferring to batteries as hold over until the transfer was completed.

The batteries were charged, the generators ready to go but the site died. A faulty relay brought the whole thing to a halt. A site tech, who just happened to be on site, was able to restore power after a couple of hours of fruitless consultations with the senior techs and high tech tweaking. This was accomplished by whacking the power transfer control unit case with a hammer.

Lesson being that even with all the care and redundancy in the world things don't always work. These systems are designed, built and operated by people. Failure is not an option. It is an inevitability. That was a very well prepared site with a lot of redundancy. I have seen a lot of other sites that were far less prepared and/or the backup systems not well maintained. Corporations often slack off on maintenance and testing of emergency systems when money gets tight.

In a major emergency cell phones may, or may not, work.

There are a few things you can do to slant the odds in your favor:
Selecting a cell service provider go with one of the larger names. Off brand providers often are using a major provider's equipment. When things get tight the people who own the equipment will favor their own customers. Smaller providers that use their own equipment often are using older and less well maintained systems. Some have problems maintaining reception when things are normal.

Also look at the coverage areas. Major providers are often localized with a majority of their network located in a specific region. A major provider with most of its towers in the SE is going to be more spotty in the NE.

Select your phone for reliability, signal strength and clarity. Good battery life is important but many of the figures published are based on the phone operating at the lowest power setting with an intact cell tower close by. One of my friends still uses a 'brick', a four pound base unit about the size of a book with a handset on a coiled cord, simply because it has a much higher power output and it accepts a variety of external antennas. With this unit he can send and receive in remote areas and work around dead cell sites.

When selecting a cell phone look for one with an external antenna connection so you can hook up a larger antenna.

Operation at maximum power to reach beyond disabled cell towers a cell phone is going to eat batteries so have a spare ready to go. If you use rechargeable batteries you need a system set up to keep your batteries charged so they are ready to go when time comes. An adapter that will feed power to your phone from AAs and a set or two of lithium batteries is a good way to start. A power adapter that will run off your vehicle power system adds options. Even a small automotive battery will keep a cell phone going for a long time.

Having some way to recharge batteries is good in the long term. Fold-up solar panels are okay if there is dependable, strong sunlight. Small hand crank generators can work but make sure you try them out. Some cheap and poorly designed units will break or wear you out after a few minutes use.

Of course having all this stuff won't do you any good if the cell phone doesn't work. Get a good quality unit to start with. Consider having a spare. Most companies will help you set it up so they both use the same number so there are no extra charges.

Consider having dry bags that allow the phones to resist 'the mud, the blood and the beer' while being used. Just to make sure, you might keep one in its skin and inside a sturdy case. Pelican makes a nice line of suitable cases. While your at it the external antenna, spare batteries, adapters and crank generator could all go into the case.

Don't forget to include a pad and pencil in your kit. Under stress the memory can fail. When communications are spotty you want to prioritize messages and write down what you have to say and get the important points out first. You might start with the number of able bodied/injured, present location, intended direction of travel (if any), and when you will try to communicate again if the connection is lost. Come up with your own protocol. Keep in mind that you may only get a few seconds and one try so carefully consider what is important. If the connection holds you can always fill in the blanks further along.

In emergencies IM messages use fewer communications systems resources than voice. In many cases people have been able to get through with short IMs when the voice communication systems were clogged. It may pay to familiarize people you are likely to call with prearranged IM codes. Send older relatives a 'cheat sheet' of common terms and codes ahead of time. Keep a copy in your communications kit. Keep it intuitive and as simple as possible.