When a number is activated and hasnt been topped up in 6 months outgoing calls are barred, after a further 6 months (12 months total) the number will be cut off from the network, the customer then has 12 more months to reactivate the number and top up to completely activate it, if the customer has not been in contact in those 12 months the number can then be recycled and sold on, however there is no specific time after those 12 months that they issue the number to be resold, i suppose it depends on how busy new numbers are going at the time