The number retention logic of giffgaff SIM cards in the UK is actually very simple, but the premise is that you first understand how it views "active users."

giffgaff is a virtual operator in the UK that runs on the O2 network. Unlike domestic SIM cards that look at monthly fees, or some EU cards that enforce recharge cycles, it only cares about one thing: whether the SIM card has been "actually used" within a certain period. As long as the system considers you still active, the number will be kept for you; once it is deemed a long-term idle resource, it will enter the recycling process.