These two previous posts are pretty much dead on. Modern cell systems are continuously adjusting the phone power to minimize interference from adjacent phones and to maximize the battery life of the phone. If your signal is weak, the phone is probably already running the transmit amplifier as high as it can.

It is also true that control channel signalling is going on continuously while the phone is on. Some of this signalling may get through even though you can't establish a voice call. It's also true that any given carrier may log this data in different ways and have differing abilities to access the information. It's not something that happens every day, so the switch techs may or may not immediately know how to do it.

The FCC has required that all carriers have the capability of obtaining location data from cell calls within a defined level of accuracy. However, not all carriers may have this equipment yet, and rural carriers are likely to get it last. GPS in the cell phone provides the greatest level of location accuracy, but not all phones have this yet. There are several location technologies that do not require GPS in the cell phone, but they are potentially much less accurate (miles).

It's true that messages may get through where voice calls can't. Voice calls require a constant data path while a packet message doesn't. Similarly, if you are in a disaster situation where the voice network is tied up, short messages might get through. In particular, EVDO (Sprint, Verizon) runs on a completely separate radio network than the voice system, and is likely to be less busy. Working with a Salvation Army Canteen unit in Mississippi post-Katrina, they were getting emails slowly while voice calls rarely made it through.

The antenna and terrain are huge issues when considering the ability of the phone to make a call. I used to have a larger phone with an extendable antenna. It tended to work pretty well. Newer, tinier phones with no external antennas make it harder to get a good RF link. If your phone can take an external antenna, it will have a much easier time getting out. You lose a lot of signal making a call from the inside of a car.

Getting the phone up high with respect to the surrounding terrain and trees will make a big difference. If you have a bluetooth headset, it could be possible to put the phone on a tall stick and make a call from the headset. Maybe tie the phone on a string and throw it up on a tree limb. My bluetooth hands-free thingy in the truck works 30-40 ft outside my truck.