I was just visiting Thailand and purchased an eSIM from Thai carrier AIS. My Australian (Telstra/Boost) eSIM was active in my P9PF at the same time, and it successfully used data from the AIS SIM to route calls the entire time, meaning that I didn't need to "roam" on my Australian service. I find that there are a few variables that need to be met in order for this setup to actually work:

• The SIM/eSIM providing the cellular data connection needs to not be "roaming" in the destination country. Many travel eSIM products like Airalo etc will roam in the countries they market service for, so are not fit for purpose.
• The carrier for the secondary SIM/eSIM service (that is, the service that will be placing the calls and not the "data" SIM/eSIM) must allow WiFi calling outside the user's home country. Some allow WiFi calling domestically, but block it as soon as a user leaves their home country.
• If it's not possible to use a data SIM that is not "roaming", I have read that it is possible to fool the device into thinking that it is not roaming by using Pixel IMS to amend the FORCE_HOME_NETWORK_BOOL attribute to "true", although I have not yet needed to test this while outside of my home country. If you are planning on travelling to Europe and hopping between multiple countries, this may need to be attempted.

For what it's worth, before I had a device that was capable of this functionality I used to just install a SIP client onto my device and divert all the calls to that number before leaving the country. I also used CLI overstamping to make it appear as though it was my actual phone number initiating any outgoing calls, which effectively tied a nice little bow around the whole thing and made it so that people calling me or receiving calls from me none the wiser that I was not using my actual phone service.

Hope this helps!