No phone calls in or out only internet 3G

Hello,

I've got this iPhone 5 totally unlocked and no blacklisted which suddenly starts to catch the signal also for the 3g connection but when I try to call, immediately it crashes the calling. If someone tries to call me it seems like it is Switched off.

It only browse the internet thanks to the 3G working connection.

Have you ever experienced something similar?

Any help is much appreciated