Step 1: Open your Phone app from ou Springboard then dial this number *3001#12345#*
Step 2: Now your iPhone will launch Field Test, you will notice that bars converted to numbers. This number is indication to your reception strength measured in decibels. The closer to 0dB the stronger your signal.
Step 3: Press and hold Power button until the power slider appear, but don’t power off.
Step 4: Press and hold Home button until Field Test program is terminated. You will find that strength numbers still existing.
You can toggle between signal bars and numbers by tap on signal strength at the top left of screen.