International wifi calling

Hi,

I am using prepaid services on my cell phone. I currently run services from two carriers simultaneously - one on an eSIM (ATT on trial) and one with a physical SIM (Mint Mobile). I have a brand new iPhone SE. Wifi calling is enabled for both carriers. I can make wifi calls to domestic (U.S.) phone numbers without problem, but not to international numbers (specifically, Western Europe).

To be more specific, I was actually on an international call for about half an hour or so, and then the service cut off and I have since been unable to get international wifi calling to work. On Mint, the error message says "you do not have enough credit to make the call" and ATT simply says that the call cannot be completed.

I called both Mint and ATT to inquire about wifi calling restrictions, but they both say there are none. I have gone through all possible trouble shooting with both, and nothing.

Further, I have tried calling with another phone that is on Google Fi (not an iPhone), and the international calls go through without a problem.

Does anybody have an idea what to do or why this is happening?

I have read in older posts here in the community that wifi calling does cost the carrier and therefore it is not free, but neither Mint or ATT have said as much to me and said there are no restrictions. I am at a loss and would appreciate some help! Thanks!