Do iPhones only lock to the carrier/network they are activated with?

[iMore Question]

I have read that iPhones themselves, whether prepaid or otherwise, do not come from Apple locked to a particular network/carrier, and that this only happens when they are activated to that network. Is this true?

 

[Ecm]

An unlocked iPhone purchased from Apple remains unlocked, even after a carrier sim card is inserted. Example. When I purchased my iPhone 7 Plus from the Apple store, I was on the Telus network. When I switched to Rogers, it was as simple as popping out the Telus sim and inserting the Rogers sim.