I've got a Nexus 4 running Android 4.2.1 on AT&T (via Straight Talk). I've noticed that sometimes at work (low signal area) instead of getting low bars (zero to two at my desk) I'll get "no service", with an empty triangle instead of the shaded in triangle with an H over it.
Moving to a place with better signal (outside, at home) doesn't bring back the signal indicator, but rebooting the phone does. I don't think it always does this, either, but I may have just not noticed. Once I even got "emergency calls only" which I've only previously seen when I had my SIM card but hadn't activated it.
The weird part is rebooting the phone at work brings back the signal meter too; the problem seems to persist until a reboot. Turning airplane mode on/off didn't seem to work. What's causing this? It seems like once it loses signal it just kind of stops looking for a signal until it reboots. Is there some way to prevent this or force it to reacquire a signal once I know I'm in an area with signal?