Thank you TGarza.

I think the confusion lies in the fact that the dual sim, dual standby became active on TWO ESIMS with the 2021 model iPhones. I believe the IMS function actually was available around 2019 or so with iOS 13. My first dual sim phone was the iPhone XR from late 2018. I was able to install an esim in Dec. that year in addition to the SIM card.
Initially there was a lot of speculation about setup and requirements for this to function. I found some notes on another forum from Jan. 2020 I made for this setup.

Anyhoo, I’m still satisfied that the US Mobile $10 “Warp” line is well worth the money for traveling. Having a worldwide voice line available for calling while keeping the cheap or included data roaming in place is awesome. And having a second network with voice & 2Gb of data easily justifies the cost for me while in the US. Great customer support is also present, with the reps active on the Reddit US Mobile subforum.