That's true. The main point is that in a whatsapp call (or other
data-based call) each user establishes a connection that they are
responsible for, regardless of what connection conditions the other user
has at their disposal. It might be WiFi, or inclusive data, or
non-inclusive data, but the main point is that each user's connection is
independent of the other's. If there are multiple users mutually
connected on such a call, each will pay, or not, according to their
individual connection type.