Sorry, wrong answer.
The signal bar on the top left of your iPhone screen will be replaced by a number, indicating the
signal strength of
the cellular signal.
You have to understand that dB and dBm are only somewhat related. Yes, RSSI does have units, and yes I did typo my last post, but they're certainly not
dB. Quick explanation: it's not an arbitrary ratio (like dB) but a ratio relative to 1 mW. The ISA
has a nice explanation.
You also have to consider that signal strength is not the only factor; there's also the signal quality
. You can be receiving a very, very weak signal, but as long as it is relatively clean (i.e. free of noise and other interference), you can still have a perfect phone call. On the iPhone's hardware specifically, the minimum value is -121; -110 is not
the "minimum". You can make a perfect phone call at -120 (and I personally have).
Think of it like a DSL line. If your line is clean, has minimal interference and crosstalk, as long as your SNR is above 6 dB (yes, this is actually decibels and not dBm), your attenuation won't matter as much.
TL;DR: RSSI is not measured in dB, is relative (not absolute) and signal quality matters more than signal strength
Edit: To answer your original question, your phone is reporting the correct signal level and you should not worry about it.
Edit 2: I remembered the name of the app I used to use from Cydia, it's called "Signal". It costs a few dollars but it's interesting to see how the signal quality compares to signal strength for any given tower.