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?