If you stop topping up and the validity period runs out, the line is cut off in stages: outgoing calls are barred, then incoming calls, and finally the number is recovered (the carrier closes the line). Once a number has been recovered, you cannot get it back. How long each stage lasts depends on the carrier. SKT prepaid lines, for example, can still receive calls for 10 days after the usage period ends, and any balance is carried over if you recharge within that window. Some MVNOs keep incoming calls alive for 15 days after the balance runs out and close the line 30 days after that. Check your own carrier's terms for the exact dates.
