I have a Galaxy Ace on PAYG. It's been great from the word go, but in the last day or so I can't make any calls. I get the dreaded "MOBILE NETWORK NOT AVAILABLE".
If I go into SETTINGS and search for "network operators" I can see Vodafone UK. If I select to connect it says : "Registering on vodafone UK..." then "your SIM card does not allow a connection to this network".
I've tried the usual : flight mode on/off, 2G only, complete reboot, even reset to factory settings. It's still the same.
Saw an earlier post about IMEI being blacklisted, but other than that I'm fresh out of ideas.
Interestingly if I take the SIM out and place it in an old NOKIA it works fine, and I could even add some top up ok. So it sounds like the phone rather than the SIM.