Typically, if you buy a phone from a carrier, it'll be locked to the carrier, whether or not you get it on a plan.
That mostly applies in North America.
Imagine a demographic where you have five different carriers using the same tech. You end up with third party sellers, mom and pop stores, boutiques, etc. all dedicated to selling phones and accessories without being associated with any carrier. Any of their phones will work with any carrier so their inventory doesn't have to be split between the different techs. They can stock up on phones that will hit every possible pricepoint and work anywhere.
Eventually, people started getting their phones from them instead of the carrier (More pricepoints, more models) until it comes to the point that the carrier is just there to provide cellphone service, not cellphones. Soon the market got so saturated with factory stock phones that if a carrier were to come out using a different tech, they would automatically lose 99.9% of the possible subscribers. So rather than lockin, they resort to prepaids.
It's no longer uncommon for a single user on prepaid to carry multiple sim cards from different carriers, usually carrying a high end phone for their primary carrier and a couple of ultra cheap dumbphones for the others, the manufacturers noticed this and came up with dual-sim phones. So now a single phone, can connect you simultaneously to two carriers. Why do they do this? It's because, with the inability to lock in subscribers, carriers now add incentives to make users call within their own network. For example, they can put a promo where between 6pm and 12am, call rates are discounted if a call is made within their network only. The closest thing i can compare it to is a McDonalds vs BurgerKing, i need to go to their website to keep track of the different rates, promos, and packages, and i presume the other users are doing it too.
Guy walks into phone store (Which is not associated with any company, carrier, or brand), sees different phones on display, and the sales lady shows them an unbiased description of what each phone can do. Now considering this is a regular joe that just wants a phone in a demographic where lock-ins are uncommon (And so is the word 'Ecosystem'), what does the iPhone have that the other phones don't? Would you choose a $1,000 iPhone 5 over a $500 S3? Aside from status symbol of course. If apple were to make an apple ad here to encourage people to buy iOS phones, that's free advertising for the competition because the moment they step into the phone store, they see all the other alternatives, and they're usually Android.
This is what the current state is in SEA, and apparently from conversations, the guys over in Europe are the same way.
