Does the Pixel 6 allow both lines to be active at the same time? That is a primary condition of the IMS feature. If so, Assign your voice line to a non service carrier. For instance, in the US if you are on AT&T, set your voice line to be on Verizon or T-Mobile. Put the line for data on the carrier where you have service, i.e. AT&T. After a few moments (1-2 Minutes Or So) your voice line should declare "No Service, Using Data" or something similar. At this point you are set up to make calls to and from your home country without charge. They appear on use statements as a WiFi call on my T-Mobile account. I apologize that the only dual sim phones I have are iPhones, so I cannot be more specific about other brands.

Why would cellular data not charge but WiFi charge??

I believe the use of the data is not charged by either the parent carrier (T-Mobile For Me) or roaming carrier (Say, Vodaphone In Europe) because the data required is so low.. The amount of bandwidth required is minimal. I have tested it and it seems to be around 16kbps, or 1Mb per minute on a voice call. The free 128kbps or 256kbps can easily handle these type data requirements. You can even do low resolution FaceTime with that. Also, it apparently uses similar technology built into iPhones at least that is employed for Facetime Voice calling.

I'm guessing that it does not require a voice channel from either carrier, so it cannot contribute to tower voice congestion.

Edited to add: I see I really didn't answer your final question about the possibility of a WiFi charge for calls. WiFi calling will incur a charge if you are connecting with a non US number. Or, if you get a call from the US and have WiFi on, it might connect via WiFi calling @ $.25 a minute.

I finally got my AT&T iPhone unlocked so now I am learning about eSIM and how to use it when I travel internationally. It seemed straightforward - if your voice carrier/plan allows Wifi Calling and you have Wifi or Carrier Data connectivity on your phone, the voice calls will work.

Reading this thread, it seems it is not as simple as I thought? I didn't even know what IMS means and had to look it up. It is basically sending everything through IP, right? Isn't that what they do now anyway even when on cellular (i.e. VoLTE)? What am I missing?

Thanks.

Yes. The "I" in IMS stands for Internet Protocol.

The premise of IMS is that with a dual sim phone you can use one line for data, and your traditional voice line then uses that data connection as a simulated WiFi service for WiFi calls. On T-Mobile plans, WiFi calls to and from 210 foreign countries are included in your monthly service with no additional charge.

For iPhones, all models since the XS & XR have dual sim capability with esim & nano sim functions. They are easy to set up, and offer various options in the Cellular Settings portion of the Settings App.

I do not know what AT&T's policy is on international WiFi calling, so if they do not charge for it, then you can use IMS for free calling in my opinion. My limited personal experience due to current travel restrictions is that you can even call other internationally roaming US phones on IMS without charge (A Round Trip, So To Speak), but I need to test this further.

Do you intend to use 2 ATT lines, or another carrier in addition to your daily voice line?

Where is that from? Last I recall if you're outside the USA, you only get free calling back to the USA.

if you are in one of the 210 free roaming countries, and have WiFi calling enabled on T-Mobile unlimited plans then incoming calls to your US number are also free.

From the T-MOBILE website:

Billing for Wi-Fi calls and messages

What you're doing What you're charged on Wi-Fi Calling

• Receiving any calls or messages
• Calling to U.S.* phone numbers
• Sending messages to U.S.* phone numbers

If you have an unlimited plan:

• Any incoming calls: No fees
• Any incoming messages: No fees
• Outgoing calls and messages that you make to U.S. phone numbers: No fees

I am reasonably certain that this feature does not work on Pixels - I have tried it in the past without success.

There is a work around for T-Mobile customers - you can use a data ESIM with the DIGITS app to make and receive calls on your T-mobile number.