Field Test mode can be handy if you’re knowledgeable about cell phone technology but it can also help you get a clearer picture of what your cell phone reception is like in a given area or perhaps you’re just somebody who just likes to tinker with things, either way here’s how it’s done.
If you’re iPhone has a recent version of iOS on it (4.1 and up), which covers most phones, then you should be able to access the Field Test Mode with most carriers.
To Access Field Test Mode:
Dial *3001#12345#*
This will allow you to look at lot’s of otherwise hidden information about your phone and it’s connection to the network. For the average user, the most notable feature is the ability to look at your overall connection to the network.
The number is in dBm and is much more precise than just looking at a set of bars. As a general guideline anything below -100 is starting to get pretty bad, the lower the number the better and -35 or -40 is about as good as it gets.