Best answer by Gordo G
"If your SIM has been secured with a PIN, you will be asked to enter the PIN anytime you place the SIM in a different device. If you enter it incorrectly three times, your SIM will automatically be blocked and you'll be asked for a PUK (PIN Unlock Key)"
You can get a PUK from one of their 'Live Chat' agents on their website. (https://www.idmobile.co.uk/).