Data only eSIM

There is no such thing as a data only esim. You can get a data only plan but a sim is just a sim. In order for you to use WiFi calling, your ATT sim must be active which means the foreign sim would be a second sim and your phone needs to be unlocked. Unfortunately, getting your ATT sim to use the data connection on the second sim to connect to ATT’s network rather than roaming may not be easily done. I’ve seen multiple posts from others trying to get that to work with no clear solution. If you can get the ATT sim to use the second sim data connection, yes, it should connect to ATT’s network and show “ATT WiFi” as the carrier. 

There is such animal as data only eSIM. I did some further Google search found a link that explain the how to use it and how to make calls even using the number of the primary sim. The article mention there might be charge for the primary carrier. But if the primary carrier is a unlimited call AT&T account, I don't think there would be a charge.

https://alosim.com/how-to-make-phone-calls-with-a-data-only-esim/

Again you may still get billed

Again, it’s not the esim that makes it data only, it’s the plan and how the line is provisioned that makes it data only but that’s just arguing semantics. If you want to call it a data only esim, go ahead.

But if the primary carrier is a unlimited call AT&T account, I don't think there would be a charge.

You would be wrong in thinking that. If your ATT sim is roaming on a foreign carrier, then any call will trigger the day pass charge. The only posters I’ve seen successfully get their phones to do what you want had prepaid service on their ATT sim which does not roam.

I can't find any evidence that I won't get charged for the call so I will wait for the opportunity when I can actually play with the esim option. But meanwhile for my particular case, I have Google Voice number as my "primary" phone number which rings on the AT&T number. I will get my calls and texts on my primary number whenever and wherever I have data from the esim or wifi.

@Shane100  If you have a phone, you can add service to the esim but it's a phone, you will not have data only service.    It's actually against FCC rules for a phone not to be able to make a 911/999 phone call which means it has to be a phone plan which has talk text and data.   What you were talking about is choosing to only use the data portion of phone service that has been added to your second Sim. That's not the same thing as data only Sim or data only service. Talk and text is still provided anytime you add service on a phone. 

But that's irrelevant to your goal... 

As for what you're trying to do, using the e-sim as a Wi-Fi connection for your other AT&T sim on the same phone, I have seen a lot of claims and even instructions on how to do it. But I have not yet seen anyone prove they have done it.

   What I have seen is a number of people who follow the directions and say in theory it should work but in practice it doesn't. I suspect those who are providing the information have not actually performed the action successfully.   Bringing a cheap old phone with you and purchasing service from another service provider and using just the data portion would provide a functional Wi-Fi connection that could be used to make a Wi-Fi call back to the United States for free. Just to cover your bases, please put international day pass on your line anyway. If you don't connect you don't get charged. If you do connect you avoid some seriously brutal High charges

And wifi calls are not permitted in

China, Saudi Arabia, India, Turkey, Israel, United Arab Emirates, Vietnam, Pakistan, Cuba, Iran, North Korea, Sudan, and Syria.

Talk and text is still provided anytime you add service on a phone. 

@formerlyknownas The OP is talking about adding a UK or German esim data only plan. FCC rules don’t apply, they may very well allow a data only plan/sim on a phone.