[usr1106]

You seem to be talking about the US. T-Mobile seems to be the only operator not having custom firmware for modems. Maybe a small remainder of the European influence in the company. In Europe national and international interoperability between any SIM and device (1) has been a thing since 1992 when GSM came. In the US operator business models were based on technical incompatibility and complete lock-in for decades.

(1) SIM locks exist in some countries. But they are commercial practices, not technical incompatibility.

[vinay427]

I don’t see how the history of GSM is very pertinent to the present scenario. At least in the last several years I’ve used phones from Europe or Asia or other networks in North America and they all just work with first-party or MVNO SIM cards in The US, from T-Mobile, AT&T, Tracfone, Mint, and others, assuming the device supports the appropriate bands (which varies a bit by region, especially for LTE).