Did you try just restarting the phone? I've come across this too before but I didn't freak out as much as you did. I found the issue to come about especially when switching from wifi to 4g. For example, I get crappy reception at work so I'm usually on wifi. But the wifi connection at work is also crappy and often the connection will drop.Now the phone will show full signal, but will give me a 'not registered on network' msg when I try to make a call or refresh my facebook feed. A restart of the phone fixes this for me. After the restart, as long as I have two bars, I can usually make calls...even though data apps may still suck. As soon as I leave work I get full signal and if my wifi had been off earlier, all my messages flood in at that time. I don't think you have to do a restart every time. I think if you give it some time it registers the network itself. But a restart may be an option if you gotta make a call pronto. Hope this helps somewhat.