I've always thought that the ban on voice calls of any kind is because people, as a rule and on average, are less sensitive to how loud their voice is in a phone conversation than in a face-to-face conversation. This is in part because the person on the other end of the line isn't there to see cues about what is too loud to be polite – closely related to the reason talking on a cell phone while driving is more dangerous than talking to a person sitting in the passenger seat. An outright ban on voice calls is much easier to enforce than a subjective rule about speaking too loudly.

Though I appreciate that you were making every effort to be quiet in your audio call (and may well have been quieter than some in person conversations on the airplane), I personally think the ban on calls is for the best of everyone on the plane.

I have no idea why the FCC or FAA should impose a ban themselves, but I am glad that the airlines all choose to do so as well.