How does that work? Does the carrier have to support each new brand of phone?
I don't think we have a system like that here in the UK. I have an iPhone 6 paid for with a monthly contract, but I was using a ~2002 Nokia feature phone recently while it was away for repair.
For example, dual SIM card phones are not offered on any phones built for the US market, since carriers make a lot of their money from financing new phones. There are also phones with three and four SIM cards made for other markets. Need two (or three or four) numbers? In the US, you have to buy two (or three or four) phones.
AT&T and T-Mobile use the GSM protocol on their networks, which is the protocol used in the rest of the world. The 2G bands use different frequencies in this hemisphere than the frequencies used in Europe and Asia. The 3G and 4G and LTE bands are different yet, but the basic protocol is the same, so if the radio works, the phone works.
Sprint and Verizon use a different protocol called CDMA which is incompatible with GSM. Phones designed for foreign markets do not work at all on their networks.
So to summarize, barriers built using different protocols and marketing limit the ability for US consumers to treat their handset for what it is (a commodity) and help lock them in to one of two carriers. This is why Apple and Samsung can get away with charging $600+ for devices that cost a very small fraction of that to manufacture using impoverished laborers.
I just bought a 2G Philips dual SIM phone made in China for the Russian market that will work on T-mobile's US 2G network. (It will not work on AT&T since they turned off their 2G network on January 1.) I plan to use two SIM cards for the two numbers I have, and to limit mobile data to a tablet for those times when I absolutely need it. Everything else (paying bills, looking for work, shopping, playing games, getting directions, etc.) will be done on a laptop or desktop.
I would welcome reform of the US cell market, but I'm not holding my breath for that to happen. They simply have too much power, and most Americans are completely unaware that it could be any different or better.
This was only true up until LTE. LTE is GSM.
US CDMA carriers right now require that you whitelist your phone's IMEI before it will activate. That means, even if your phone is compatible with all bands used, you can't just pop a SIM in and have it work. That may change once VoLTE is everywhere, but it may not, because carriers like making it hard to bring your own device, so you'll buy it from them instead. Verizon has some restrictions here, due to their purchase of the 700mhz LTE spectrum, but they still make it difficult.
Actually there are plenty of dual SIM models available in the US from US companies that are 100% compatible with US carriers, you just have to be willing to open a browser and shop for five minutes. I am currently using a Microsoft Lumia 650 dual SIM phone, purchased from Microsoft and shipped to me in Georgia (US) from California. It fully supports both AT&T's and T-Mobile's networks, and consequently all MVNOs of those networks. Until I gave her my Nexus 6, my wife was using a dual SIM Blu Android phone, sold by a Florida based company and shipped from the US. Again, the phone supported all US GSM bands.