The sim should be the correct type for the account.
PAYG and contract sim cards are different, so ensure you get the correct type.
It will be printed on the sim packet.
The number is set by the sim, hence having the correct sim type.
For customers on the PAYG the message centre number is +447802092035.
For all other O2 customers, the message centre number is +447802000332.
No problem @GDavies
Happy to have answered your questions