I had been hoping to determine which GSM phones allowed access before buying one. Thus, I do not own any such phones yet and cannot try the few suggestions I have found.

To go back to the 0-7 bars on Nokia phones foe RSSI, do you know where the breakpoints between bars are in dBm? I worked out experimentally for a CDMA LG phone that I could reach field test mode on, that for its 0-6 display: No Service -80.

So for field test, I really like Windows Mobile. It’s old and decrepit, sure, but there’s an official sanctioned application that exposes more than you’re going to get with almost any of the other solutions. After that would be Android, which does an awesome job and has tons of third party applications that allow you to monitor signal like Real Signal or even in settings -> about, or widgets. Next in line are manufacturers like Nokia which have field tests exposed with dialer codes. I always search “[phone] dialer code” and get back a ton of stuff.

I’d actually put iOS in last place when it comes to field testing and signal reporting. In iOS 3.x, you could get to it with the *3001#12345#* menu very easily, but that was removed in iOS 4.0 and later replaced with a gimped version that only shows numerical signal strength (tapping on bars method), none of the other info. There are very few options outside jailbreaking and using a Cydia app called “signal” to show strength.

If you know of any sources of info on field test mode on GSM phones (including how to read the displays), I would greatly appreciate learning how to get to them; Nokia 2720, especially.