There seems to be a new trap in the USA. I have been there for the last month.
Most countries in the world you go to the store, buy a SIM and away you go.
Both companies have similar deals, $30-$50 all the calls you want and 5-10 gig data, 30 days etc.
ATT and T Mobile both told me they will not guarantee a SIM will work in my unlocked Australian phone...the reason is the system compares the IMEI number of the phone and it will eventually figure out that the phone was not sold in USA and is not "on their list". I visited several stores and all staff said they have had tourists come back angry after a few days when the SIM stopped working or in many cases they couldn't set up the phone at all...after they crack open the SIM and charge you for it of course.
A work around is to call ATT (in my case) and get your phones IMEI number added to the system, then it will not be deactivated...this only took 35 minutes of transferred frustration...
Problem 2 was my S10 (JB Hifi Aust stock, not a grey import) was unable to get 4G anywhere due to different bands, or bands missing from the new Samsung, I don't recall this being a problem on my last visit 6 years back...so better check what bands there are.
On coverage it was very poor or none around the Grand Canyon, Bryce, Zion, and Monument Valley. Verizon (on old CDMA network) is king out there. It worked ok, only getting 2G but, in most other cities.
Would appreciate any other recent feedback...seems a generic data only SIM may be a better deal.
Also unsure if iphones have same drama re 4G?