One of the protections enforced in my country is this – for 24 hours after mobile number porting, all incoming/outgoing sms are blocked. And on both the current sim and new sim, notification sms are sent to inform the user that mobile number migration is occurring. This gives you the opportunity to notice and put a stop to it if it was triggered fraudulently. But of course there are corner-cases to this. If you are personally targeted in the meatspace, then all bets are off.