I don't know if this will do any good, but somebody might find it interesting.
http://www.cellreception.com/towers/

It is a mapping of cell coverage.

Something to think about re: long-wire antennas:

In order to function optimally, antenna's work best when they are "tuned". That is, a wire length is selected that 1) provides electromagnetic resonance at the frequency of operation, and 2) offers a feedpoint impedance that matches the impedance of the transmitter output.

Failure to do so may result in 1) degraded performance, and/or 2) possibly damaging the output circuitry of the transmitter rendering it non-functional. This may be more critical at microwave frequencies.

Actually, I'm not certain how critical the impedance-matching factor is these days. It may have been largely resolved in modern integrated circuit transmitters, but this resolution may actually involve some protection circuitry that automatically reduces power in cases of impedance mismatch in order to prevent cooking the micro-electronics from reflected power from the antenna. If so, using the regular cellphone antenna may provide the highest and safest signal output.

I know some phones have the option of using external antennas.

While they may not be much longer than the phone's native antenna, it does allow for one to place the antenna somewhere that may get better reception (say, up a tree or something) without requiring the user to place themselves in danger just to get a signal.

With that said, if I actually needed a cell phone that I could somewhat rely upon in remote places....I would probably be looking into a satellite phone, or at the very least, a modern bag phone (which is still allowed to operate at 2 watts). Typical consumer phones tend to fall short when it comes to rural/remote use.

At the risk of being called old again, does anyone remember the Motorola bag phones? Now there was a bear whomper!

Not to mention five watts of power. Here in Texas, the farmers resisted the change to digital for so long, their phones just finally quit working. Local carriers kept the legacy system going for several years beyond their original cut off date because the farmers/ranchers just wouldn't give up the old mounted phones in their trucks. Those old unite would get a call out much farther from a tower than the newer 2G and 3G handsets.

I've use a Wilson external cell phone antenna on my old "smartphone", (worst cell phone I've ever had the occasion to use by the way but I digress), it definitely helped when using the phone up in the high deserts.

I now use a Blackberry, no external antenna connection on it. After talking to some sales reps from Wilson I learned that the only way to get the benefit of an external antenna with it is to use the booster amplifier along with my original antenna.

Couple of hundred bucks for the booster, haven't purchased one yet.

I hiked the Grand Canyon last weekend. I did not take my cell in with me but others did. I made a call and text from the 1.5 mile resthouse on Bright Angel using a friends phone.

Texts work on less 'bandwidth' than calls but you still need an active signal. IME, I have tried many times sending text just outside a signal area under the impression it would continue to resend until I walked into the signal in the next ten minutes or so. This is not the case with my phone. It alerts it cannot send it and goes to drafts.