Hi @EstherK , you would need a new Sim card when changing from pay and go to contract but you can definitely keep your number. Is it definitely currently pay and go or is it a Sim only contract you currently have because if its a Sim only contact you wouldn't need a new Sim to change your tariff.
Switching your number from Pay Monthly to Pay As You Go
We can take care of moving your number and even arrange for your new Pay As You Go sim. Get in touch by calling us on 202 free from your O2 phone, or on 0344 809 0202 from any other phone. We'll do the rest.
If you're on O2 Refresh you can only move to Pay As You Go once your Device Plan is paid. If you're not on O2 Refresh but still in your contract minimum term, you might need to pay a termination charge, and our Customer Service team can talk this through with you. You’ll receive your final Pay Monthly bill 14 days after you move to Pay As You Go. You’ll need to settle any outstanding balance.
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