Can't reproduce this issue.
Not that I want to, but I can and do reproduce this once in a while. The root cause is that the CoDi stays on when holding the Cosmo to the ear, making it
very likely that some part of the face will touch something on the CoDi. Exactly what gets touched probably depends on how the Cosmo is held and the precise shape of the users face. In my case, some part of my face seems to fit the mute icon just spot on.
Now, there IS an icon on the in-call CoDi screen to turn off the CoDi, but it only makes the CoDi go dark. As soon as you touch it, it wakes up again, ready to be face-tapped. I'm hoping some future CoDi update will improve on that, say, by requiring a swipe on the CoDi to wake it up again. That should be pretty face-proof.