iPhone Signal Strength
To determine received signal strength on the iPhone, open your phone app, type in *3001#12345#* then Call.
The signal strength bars will now be replaced temporarily by the numeric RSSI in decibels.
Once you have launched this feature it will stay even if you receive phone calls, but if you close the phone app it will revert to the bars indicator.
It appears that this command works on iPhone 3, 4, and 5.
What the numbers mean.
-50 dBm is near perfect
-150 dBm is impossible.
-50 to -85 you will generally get decent service
-85 to -100 not so much, but usually livable
-100 to -120 look into boosters and amplifiers or some sort of microcell if you have decent internet.
-120 to -150 change providers or look into satellite options