After activating your new SIM card, by making call, the mobile number is then given 30 days airtime. To avoid the sim expiring, a top up must be completed during that time. Your new sim will now be given 60 days airtime after this first top up and every top up following.
It is important that you top up your Prepaid number so that it stays active on the system. Otherwise, the number will expire. Once the number expires, it will then be recycled.
To prevent your number from expiring, continue topping up.
Note: Credit U transactions add no airtime.