Many (or all?) US carriers require VoLTE capability. Mint is one example, a couple of years ago they wouldn't activate in a Samsung A8 (failed Mint's IMEI checker, though Telus IMEI checker said it was VoLTE capable). Right there I was already confused. But we activated the Mint SIM in a Samsung S20 5G, which both Mint and Telus agreed was VoLTE compatible (even though there's nothing in Settings about VoLTE). It worked well on Mint.