iPhone 4 no longer a phone!

My wife's iPhone 4 has stopped working as a phone, everything else works fine. It keeps giving a "simcard error" message. She has gone as far as doing a factory reset but that did not solve the issue!
Any ideas please?

Couple of things you might want to check, basically eliminating one unknown component at a time:

• Have you confirmed with the network that the card has not been de-activated, expired, etc?
• Does the SIM card work in any other phone?
• Have you tried a different SIM card in the same iPhone?
• Was the iPhone unlocked, and if yes - was it by the network and/or Apple, or by someone else?
• If it was unlocked, and not by Apple, have you updated the iOS recently?

That's besides a few simple/obvious ones, like:

• Was there a liquid spillage?
• Can you still eject the SIM card holder from the phone?
• Has the phone been dropped?
• Try to remove the SIM card, clean & reinsert
• Set cellular provider to automatic, then to manual, making sure the correct network is selected, then back to auto
• What do you hear when you try to call the SIM number?

Try removing and reinstalling the sim card. I have had that happen a few times with my 5C and it fixed it.