We call our daughter in London every few weeks via FaceTime Audio using our HomePod. The HomePod routes the call via my iPhone. It works really well, or at least it used to.
In the last few months, after 12 minutes or so, the call quality deteriorates and the call eventually fails. I suspect that it is something to do with IOS 13 on my iPhone, as I believe it did not occur till I updated my iPhone to IOS 13, and before the HomePod was updated from 12.4.1
I do not believe the cause is related to our WiFi or broadband connection, as calling from my iPad Pro works perfectly. I do not believe the problem is with my daughter’s end of the call for the same reason.
We did have one successful call via the HomePod on 17 November after all the updates, but the problem returned on 1 December. It makes me think that Apple may have first fixed the problem in IOS 13.2.2 released on 8 November, then unfixed the problem with 13.2.3 which was released on 19 November.
The iPhone (iPhone 8) and HomePod are otherwise working really well.