By just going through these steps you’ll be able to see your signal reception of your iPhone in decibels (dBm). Normally the value should be between -60 and -90 dBm. Where I live the strength is only -95.
First you need to dial *3001#12345#* and press call.
Now you’ll enter “Field test mode“. When you press the home button you’ll see the default view with the strength bars so don’t press your home button yet.
Next hold down the power button until it says ‘slide to power off’ but do not power off.
You just need to hold your home button in until the app closes and your iPhone retuns to the home screen. Now it should still show the signal strength in dBm.
To undo these steps:
dial *3001#12345#* again and press call and just press the home button shortly.