It should be both. Modem would determine if you can be connected in the first place and how , then the device,.to see if they will allow special features to use on the network.
This is just my “educated” guessing of course, based on my experiences w 2 major carriers and basic reading
My concern is even bigger for some carriers, once @nicole.faerber stated that there are no plans to certify devices. Why? Sony XZ1 compact had the same Qualcomm’s chipset (w modem) (covering 95% of all US LTE bands )as it’s counterparts from other OEMs and it never got to activate VoLTE on ATT, with all tricks tried. Similar situation with other devices as well.
On the other hand , there were numerous unlocked phones not on any list able to use that important feature.
The biggest market should be US. 700 is a lot of money and for US customers they offer 12 mos to pay off at no interest. And I assume that ppl US haels a higher % of ppl interested in protecting privacy at this price.