A friend recently got a (carrier-supplied) phone and has been complaining about how it would often have no reception despite showing a good signal; taking mine to the same areas on the same carrier and doing a comparison, mine was indeed showing no bars on the signal indicator. The difference is, mine predates this stupidity, and I can also see the details in the MTK Engineer Mode app, which shows the actual signal strength --- it was around -140dBm when it was showing 0 bars.
The signal strength measurement is actually standardised: https://en.wikipedia.org/wiki/Mobile_phone_signal#ASU