[mmac_]

Chipset makers, probably correct but there aren't that many companies that make them. Depends on the analogue front end if they support the new frequencies.

I know the 3rd party module makers do tailor their modules to regions and the 'world wide' models can be quite pricey. On the region specific versions they don't include some of the hardware to physically operate on frequencies they don't specify. Sounds crazy, but a few cents here or there is worth running multiple SKU's.

There are probably other issues such as radio compliance and patents that also prevent an upgrade.

The VoLTE upgrade isn't so surprising. For a long time we had 3G/4G modules however the mobile networks didn't support VoLTE so everything would fall back to 3G. Once everyone got their act together, a quick firmware upgrade and VoLTE worked perfectly without adjusting the hardware.