WiFi calling

Connecting to a new Wi-Fi with passkey, your phone would have to be in airplane mode to disable cellular, and then you sign into the local Wi-Fi at your accommodation. Then yes Wi-Fi call or text back to the USA is free. The connection will be as if you are still in the USA.  

It is also recommended that you have international day pass on your line as a safety net..  just keep your AT&T SIM disabled with airplane mode or turn the SIM off to avoid charges.  

Pay per use charges are extremely expensive, you don't want those

I’m confused because I thought I had to be using a cellular plan in order to use Wi-Fi calling.

OK strike that question. I now understand that the airplane mode will  enable me to do Wi-Fi calling through my cellular plan. if I want to use this option with my AT&T plan, I will turn off the local eSIM And switch back to the AT&T eSIM. Can I switch between sims while I’m in airplane mode?

I don't see why you couldn't switch SIM while in Airplane mode.

However, do be sure that your phone is not carrier locked to AT&T, or you will not be able to use another SIM.

Definitely be in Airplane Mode when switching SIMs. Any cellular connection on the AT&T line could incur the IDP charge. 

FYI I use the local SIM for Apple Pay on my iPhone. My Apple Watch using Bluetooth will also make payment functions. You should be able to use the similar function on your phone. The last 5 trips to Europe I hardly paid with cash. 

If you’re planning on using WiFi calling then your AT&T sim (or eSIM) needs to be in the device. I am not an Android expert but if you’re on Apple. You can dual sim so you’re using your local sim for cellular calls / data but then use WiFi calling for calls back to the USA. 

As you can see from the screenshots below. I’m roaming in the UK on the ‘3 UK’ network but when I’m using WiFi Calling, it changes to AT&T WiFi.

I make sure I’m on WiFi calling before making / receiving calls from the States.

Yes I will have both eSims in my phone.  I’m planning to turn off the AT&T one for the most part, but I want the option to turn it on to use the free Wi-Fi calling through AT&T. When I use it I want to be sure that I don’t incur any ATT charges  because they could be exorbitant. I do think it’ll be a good idea to have the international pass active just in case that happens. But it would still be a $10 charge that I didn’t intend to pay. so I’m trying to make sure if I do use my AT&T I do it properly. it. Sounds like if I keep the phone on airplane mode while I do it I shouldn’t have a problem 

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I’ve just tested it for you here in the UK to check it’d work and it did. I put the phone into airplane mode, re-activated the WiFi and in seconds AT&T appeared. So you’ll be good. 

so is it a bad idea to leave Wi-Fi calling on all the time? Does this use more battery power?

Not at all. Lots of people use WiFi calling all the time to cover them when coverage is bad.