Doesn’t the Sidekick phone have a manufacturer SN and a SIM so the phone knows.
I just went through this and the old Motorola RAZOR knows who it is. The SIM and the phone SN both can be blocked, at least that is what the carrier said. I bought the phone outright and had to take the proof of ownership(receipt) down before they would turn it on.
I am not so sure it’s as easy as the article says it is ...
Most phones are “locked”, meaning you can’t switch out SIM cards w/out the carrier authorizing it. However, those phones can also be “unlocked” which does allow for easy swapping of SIMs. This is useful when you go overseas. If your phone is unlocked, you can buy a local SIM card and get cheap rates while there. Once you’re back home, swap out the foreign SIM for your regular carrier again.