Re: A side-effect
>Your experience with handsets is not mine at all. ATT and T-Mobile phones have each other's bands quite well covered (if not completely).
It was more of a problem with the early 3G and 4G phones - I seem to remember it was a problem that arose with one of the iPhone releases where if you travelled between say Europe and SE.Asia then you would need a particular variant, which wasn't necessarily the variant on sale in your country. However, even a few years back not all phones sold in the UK for the UK market handled all the frequencies being used by the four UK operators - with each operator selling a variant which had a selection of radios chosen to get the best out of their network plus a few key frequencies (ie. the 2G frequencies) to enable roaming albeit with limited functionality.
With 5G and the proliferation of frequencies, I suspect similar will apply: phones sold in a particular region will have variants that 'work' on Provider X's network and the agreed roaming frequencies. However, the buyer will need to investigate the spec's to determine the extent to which an unlocked variant will work on other Providers networks and the extent to which international roaming may be supported.