@OvEr_KiLLAs soon as you change tariff it bills you so when you topped up £10 it will have taken the charge instantly.
If you want to check this with Customer Services then you will need to contact them on 4445
Current Phone: Sony XPeria Pro-I and Motorola Razr 5G
I also have a Planet Computers Gemini
I work in IT as an Enterprise Messaging and Collaboration Tools Administrator (Office 365, Slack and much much more) and manage hardware both Mobile and Desktop / laptop, my desk is a bit like PC World
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We are all customers and dont have access to any o2 accounts