Anyone who has a dual sim phone, I recommend you investigate the Xfinity Mobile service. You'll also need to have a Comcast account though. Anyway, for about $1.XX per month (Taxes & Fees in Oregon) you will have a second line & number on Verizon towers with included unlimited calling and texts while in the US. This is the total cost unless you use over the included 100Mb of data on the line per month. The fees are $12 per Gb if you exceed the included data.
They claim over 200 roaming countries, and while roaming, calls are cheaper than my included T-Mobile rates by a decent amount. A few examples: Calling from the USA TO France/Germany/UK/Italy are $.05 a min. FROM those countries = $.10 a min. Some other countries cost more than the T-Mobile $.25 rate though, so you have to check each one. They have a page where you simply type the country name into a box and get the rates, just like T-Mobile. BTW, you do not want to call these countries from the US using a T-Mobile line, as they can cost $3.00 a minute! The Xfinity service also has WiFi calling available, and while roaming, calls to and from the US are free, just like on T-Mobile.

I'm testing this service on my iPhone Xr with esim now here in the US. I have my regular voice line set as my data line and Xfinity is set as my voice line. I have data sharing switched off, so as not to incur any data charges with Xfinity. I haven't really found any negatives so far except that iMessage doesn't seem to work reliably, but my texts still appear normally since both lines are active.

I generally get great local coverage with T-Mobile, but on the rare occasions when it's weak, I suspect this will be a great fallback insurance for just pennies per day, and perhaps slightly less expensive while roaming overseas besides.

It's fun to experiment, and I'd like to hear from others who may have used this internationally.