Vectone, the sister brand operated by Mundio, made the same change earlier this year.
All operators require that you you use your phone at least once every 6 months, though some have more stringent requirements (e.g. C4C 30 days, see my web site for more). Delight and Vectone are the only operators who will void your regular credit even if you use their services. Victory Mobile were even worse but they are defunct.
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Pete Forman
https://payg-petef.rhcloud.com/