OK, let me explain. When you either initiate a wifi call or take a call via wifi calling you are connected to an ambiguous IP network, but the call traffic is transmitted to a traditional telephone carrier. This isn't a problem if both parties are in your carrier's local area of coverage, but if you are in a land that doesn't have direct AT&T network access then that other carrier may induce a charge, like roaming. In addition to this aspect of wifi calling because your means of access the AT&T network is via a pure IP network and not something like VoLTE then they cannot determine your physical location and as such complicates the E911 service.
Scenario:
You are up in your cabin in the woods and the Internet service is provided by someone else other than AT&T and you have zero AT&T Wireless coverage up there. Let's say it s fishing trip but the wife stays at home and the two of you decide to use your AT&T Wireless service phones. Your access to the network is 3rd party over IP, hers on the other hand directly and traditionally on AT&T's network. The interconnect should appear as if you are on AT&T network on both ends, but they have zero control of that 3rd part Internet provider. I can imagine some could setup a metering based on protocol-based traffic and charge their customers but I doubt that is happening. Now, on your end it doesn't consume your wireless data, and not in her end either.