Are you on a 3 branded rom or a stock one? And if you go to Network Operators and search all networks, what does it show up? On mine currently it'll say "Search failed, please turn Mobile Data OFF or hang up the call and try again.'
Sent from my HTC One using xda app-developers app
All networks search in the attached image: (got the same message as you however when mobile data was enabled)
Sent from my HTC One using XDA Premium 4 mobile app