Entering Field Test Mode on iPhone (iPhone 3G and later on iOS 4 and later)
This is how to read these numbers: Anything above -80 is considered full signal strength. Calls usually start dropping or having bad connection around -110 and -120. Some users might see -130 before actually dropping the call.
Eg. If you have signal strength of -50, you have a great carrier reception. -95 is not bad. -120 is when you better get ready to say bye as that call might drop at any moment.
How to keep numbers setting
Field test mode doesn’t last and you will be out of it when you lock your phone but, if you want to always see signal numbers rather than bars/dots then go through following steps.