FaceTime Video call switching itself out to a regular carrier call

Is it possible that FaceTime can switch into a regular call when you go from Video to Audio clicking the Audio link?

I know that the Knowledge Base says no, but it seems to have happened, so that I ended up going from a free IP call on FaceTime to a very expensive call on my carriers network.

When I tried to repeat the experience, I went from FaceTime Video to FaceTime Audio, with no such problems.

Then just to test further, I tried to make a regular, carrier call on my iPhone Phone app during the middle of a FaceTime Video. This ended up stopping the FaceTime Video, and making a separate call on the Phone App. And in the Phone log, it showed a FaceTime video and then immediately after that, a separate line for a regular telephone call with the carrier.

But for my problem call, there is no record whatsoever of the FaceTime Video starting up, but only of the regular telephone call, starting at the time that the FaceTime Video started. Moreover, the Phonelog in the next line shows a FaceTime video cancelled at the time I click the Facetime Audio button.

What is going on here?