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Were you ever contacted by a debt collection agencies that Vodafone may have sold the debt to ?
If not then give Vodafone a call on their landline number in the following link. If it won't accept your old mobile number when prompted then ignore the prompts and on the 4th prompt it will.change to say press 1 for Payg and 2 for pay monthly @ChaLou13
If you get stuck then let the Vodafone Social Media Teams here catch up with your thread.
Paying off the debt should change its status on your Credit File to settled but the debt would show for 6 years regardless.
Current Phone > Samsung Note 9 _512Gb > Model: SM-N960 Hybrid-Duos.
Samsung One Ui / Pie.
Samsung Gear s3 Frontier Watch.