people cannot just use any phone they want, it needs to be a specific model supporting their network carrier, especially the network bands.

No, you can get phones that will work with any carrier's frequencies. There are some differences in phones and carrier frequency support, but there's loads of overlap -- a phone will be able to talk to another carrier's network. They have to for roaming to work.

What you may be thinking of is phones that are "locked" to a phone provider's network, which is very common when getting a phone. Basically, you get a phone plan and receive a phone from the carrier at a price that loses the carrier money. The carrier then goes and makes up the difference with higher monthly rates on your plan than would have normally been the case -- basically, attaches a small loan to the plan. It lets phone companies have a low up-front price. It's questionable as to whether this is a good financial idea for consumers, but it's not a technical limitation. As I recall, there's some regulation that requires carriers to unlock carrier-locked phones upon request after a period of time.

kagis

Yeah.

https://www.fcc.gov/general/cell-phone-unlocking

Postpaid Unlocking Policy. Carriers, upon request, will unlock mobile wireless devices or provide the necessary information to unlock their devices, for customers and former customers in good standing and individual owners of eligible devices after the fulfillment of the applicable postpaid service contract, device financing plan or payment of an applicable early termination fee.

Doesn't affect me, because I've only ever purchased unlocked cell phones.