So, they introduced a number portability policy. If another carrier has better signal where you are, the carriers jointly entrust a portability database to the telecom technology centre. Your old number stays yours; people dialling it just pay a bit of roaming but still reach you. With your existing number ported, there’s no “new number” problem — you’re back to a positive-sum game: as a carrier, to keep you renewing I have to give you genuinely better signal; there isn’t much else I can do.