Phone number recycling is perfectly legitimate and governed by the relevant ComReg regulations. I'd link to them if the ComReg made it easy to find them but it doesn't.
If you don't adhere to the service provider's terms and conditions for keeping a number/account active (for prepay this usually means topping up by some amount over some period of time) then your number will be taken back and "recycled".
If you don't adhere to the service provider's terms and conditions for keeping a number/account active (for prepay this usually means topping up by some amount over some period of time) then your number will be taken back and "recycled".