Seen quite a few people complaining about losing their credit. Despite making chargeable call etc when required.

Appears that they changed their T&C without telling customers. New terms are

12 ii        If any call credit is not used within 90 days, (including any credit that has been allocated to you by a Delight administrator) Delight will place your My Delight Account on inactive status and you will not be entitled to a refund of any remaining credit stored thereto.

https://www.delightmobile.co.uk/terms-and-conditions

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/