Sorry, wrong answer.
The signal bar on the top left of your iPhone screen will be replaced by a number, indicating the
actual relative signal strength of
your device 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.