CcMaN:  I've got a Motorola Edge 20 Pro that I purchased through PBTech, I've tested this on Spark, One NZ and 2degrees, 5G works on all, but VoLTE works only on 2degrees. The handset isn't even parallel imported!

 

• PB Tech Parallel Import Phones
• That means that 2Degrees VoLTE profile is in the phone, Spark an ONE NZ Profiles are not

All carriers make VoLTE profiles available to manufacturers its up to them to put them in the phone

AT&T explicitly list which phones work on their network in the USA

These devices are tested and approved for the AT&T network (att.com)

If you look at the example at the top of that page two sub models of the Samsung S20 will work and two will not

So, someone roaming to the USA with the two models that are not supported are going to have problems.