After each new SIM card is activated, you've got 30 days to Top Up in order to ensure you keep your number. To keep the number & SIM active, simply top up.
If you don't top up during these 30 days, the number will expire. Once the number expires, a 3 month grace period is given, following that the number & SIM will be deactivated and recycled.
Note: Credit U transactions are not counted as a valid top up.