I recently went on a trip to the UK and used Travel Pass every day. I understood that there would be NO charges other than the daily pass unless I called another international country (which I didn't. When I got home, I had three calls for $40 total in the middle of my trip that originated in my home town on a land line! Obviously, that was impossible as I was in the UK. I called Verizon Customer Service and they figured it out right away, and took off the charges. Can you figure out what happened? Partial answer below.
On these three occasions, I had turned on a home-based VPN for some web browsing and didn't turn it off before the call. As a result, Verizon thought that I was making a call from the US. This isn't the complete answer though, because the bill said that the call was from a US-based landline! I do have wi-fi calling turned on, but a wi-fi call shouldn't result in charges at all. So, I don't have a complete answer, but I can recommend that you turn OFF your VPN before making calls while traveling internationally, especially if you have wifi calling assist turned on.