I have really poor reception at my house (because of my house's construction) so sometimes just a few dots (on the top left of the phone) as indicators of signal strength isn't enough.
Here is how to enable a full numerical signal strenth display on iOS which is very handy to know if your chances of having a nice call are high.
Open the Phone.app and tap on Keypad
Type *3001#12345#* (stars and all) and then tap on Call
As soon as you tap the Call button, the Field Test app will open. You’ll notice that instead of the standard reception dots in the upper left-hand corner of your screen, you’ve got a negative number indicating the decibels referenced to one milliwatt (dBm).
While in Test Field app, hold down the Sleep/Turn Off button until the Slide to power off screen comes up. Now, instead of turning the phone off, release the Sleep/Turn Off button and hold the Home button for a couple of seconds until it will return you to the Home Screen.
That’s it, now your signal will be displayed numerically. You can tap on it to switch between the numeric and dots display.
How to read the numeric display:
There’s a whole lot to say here, but let’s keep it simple:
-50dBm: perfect reception
anything greater than -100dBm: decent reception
anything less than -110dBm: you pretty much have an expensive iPod Touch