Definition
In Korea, mobile number portability is a telecom policy and technical system that lets you keep your phone number while switching to a different carrier (the company that provides your service). It used to be that changing carriers meant getting a new prefix or a whole new number. Now you can move to a cheaper or better carrier without that hassle.
Why did your number used to change when you switched carriers?
In the early 2000s in Korea, the first three digits of a phone number told you which carrier the person used. For example, 011 and 017 were SK Telecom, 016 and 018 were KTF (now KT), and 019 was LG Telecom (now LG U+). Each carrier owned its own block of numbers.
So if you switched carriers, you had to tell every friend, family member, and coworker your new number. That was such a pain that people stayed put even when they were unhappy with the service or the price. When customers get stuck with one company like this, it's called the lock-in effect.
To encourage healthy competition and give consumers more choice, the Korean government began phasing in number portability in 2004. It arrived together with the move to a unified 010 prefix. From then on, a number stopped belonging to a carrier and became something you carry with you.
How does your number stay the same when you switch carriers?
The secret is a giant shared address book that all the carriers use. Its technical name is the centralized database run by the number portability management center.
In the old days, when a call came in, the network looked at the prefix and sent it straight to that carrier. Now, when someone dials, the system first checks the central database. It looks up, in real time, which carrier's network the number's owner is connected to right now.
When you apply for number portability with your new carrier, your contract with the old one ends automatically, and the central database is updated right away with your new carrier. So when someone calls your old number, the network's switching system finds your new carrier and connects the call without a miss.
How is it different from a device change or a new subscription?
When you buy a new phone, there are three terms at the store that are easy to mix up. A new subscription means getting a brand-new number and signing up with a carrier. A device change means keeping both your carrier and your number and swapping only the handset.
Number portability, on the other hand, means keeping your number and your phone while changing only the carrier. Of course, plenty of people switch carriers and buy a new smartphone at the same time. The key point is that your number stays, but your carrier changes.
To be more precise, any early-termination penalty you owe your old carrier doesn't disappear. It shows up on your final bill from them, and you keep paying off any remaining phone installments to them too. Also, the moment you switch, the long-term loyalty discounts and membership points you built up with the old carrier are gone, so it pays to compare carefully before you move.
Common misconceptions
To port your number, you first have to call your current carrier and cancel your service yourself.
Never cancel with your old carrier first. If you cancel on your own, you lose the right to that number, and porting becomes impossible. When you apply for number portability with the new carrier, the old one is canceled automatically in the system.
Where you meet it
After your contract ends, you move to a cheaper budget carrier (an MVNO) and keep your same 010 number.
When you buy a new smartphone, you keep your number but switch carriers to get the switching deals and subsidies.
In one sentence
Number portability lets you keep your phone number and switch carriers freely.