Having the same problem. My voice cuts out all the time while using facetime, skype, whatspp calls and even phone calls but only while using the speaker.
It is a problem of the noise cancellation that is EXTREMELY sensible.
if the other part in the call produce the minimum sound while you are talking your voice will cut out, i have tried it and that is the problem, and it is because the noise cancellation. If the other person is muted, your voice won’t cut out, you can try it.
I have gone to apple multiple times and they can’t find any solution, they even gave me a new Iphone 13 pro and the problem is still happening. Finally they told me to wait for a software update…
Basically I have a phone that cost 1000$ which I can’t use for videocalls or for phone calls with hands-free…
I hope that apple will fix it soon…