Yeah, I think you will. If the only thing you did to get it working was the Network to 'Manual' then it's definetly something wrong in the 'phone' routing database.
How old's your SIM, I had an issue years ago, becuase I didn't replace the SIM with the new one they sent (it worked, why should I change it), and then by the time it finally stopped working, the new SIM had issues in the "databases", so it took a technical call to something like 3rd line support for them to hack around in the databases and work out a mismatch in the numbers. 1st line are to deal with the mundane, sadly you have to go through that (multiple times) to get onto someone technical, and then (like me) get lucky with someone who took the time rooting around multiple systems working out why 101 did not equal 101.
The only other thing I've got is a full backup, a full wipe and a restore.
Edit: or you could just try a Network reset: Settings -> General -> Reset -> reset network Settings.
IIRC this will lose any Wifi config's setup as well.