Could you have used services that are charged from your cash credit balance?
E.g. MMS messages, international calls/texts, calls to special numbers such as 1850/1890 numbers etc.?
If that doesn't explain it then contact 48 support and ask them where your cash credit went.
Hope this helps.
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I am not a 48 employee - if you need to contact 48 support you can do so by
chat or
email ([email protected]).