Iphone 4, can't make calls out.eh

Ok so I just bought a new iphone and I set everything up, but trying to ring O2 (my network), I couldnt contact them, as soon as I ring, It says calling, and immediately, call ended.
It does the same with every number, so I'm wondering is there a fix… Its literally 3 hours old: S
I have restarted the phone. I have full bars network, and everything else works a dream.

Any help please, would be pretty good.thanks

Added (1). BTW, i can receive calls perfectly?

Try to place a case on it, probably antennas

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