Usually a phone only asks for the pin if the security system has been enabled. This would also have required you to set a new PIN which would be different from the default one.
If you can't remember and the default doesn't work, you will need to obtain a new sim from an O2 shop and do a sim swap.
I have no affiliation whatsoever with O2 or any subsidiary companies. Comments posted are entirely of my own opinion. This is not Customer Service so we are unable to help with account specific issues.
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