This happens to me occasionally. One thing you can try when you have no audio on a call is hitting the Audio button on your iPhone. A list of speaker sources pops up:
iPhone
Speaker
CarPlay (or something like that--not looking at it right now)
When I can't hear someone on a call I find that it is set to iPhone. Choose CarPlay.
That has corrected the problem it for me when it happens. There's also a way to access the output source while trying to play music but I'm not sure what the steps are. Basically, the iPhone is not putting out to CarPlay, but instead to its tiny speaker that you listen to when it is up to your ear. Change that and you fix the problem.
Not sure how to keep if from happening, but this is a fix that might work for you.
This happens to me regularly (more regularly recently) and it is the biggest pain point with the vehicle - and a big pain point. CarPlay will be connected and running but no audio coming through. I do a reset with the bottom left and bottom right buttons on the steering wheel once or twice and it usually comes back, but this takes as while for the reboot. AND one of the two buttons is volume down and it ALWAYS takes the volume down to zero. So I have to up the volume after the reboot to even know if it worked.
Mine does this every so often too, don't believe its a phone issue as my wife's car never has this issue with my phone via wireless CarPlay.
Anyhow quickest fix I found was toggle the wifi On (disconnects CarPlay) tun bluetooth off then on, disconnects from truck. then select your phone on sync and select CarPlay. The reconnect has so far fixed the auto issue for me everytime. I have not had to reset my phone.
I got very tired of the issue, CarPlay just connecting a portion of the time. I disconnected it, reconnected and chose wired only. It's only a slight pain to plug the phone in each time, but it has relieved the stress of me cussing most every time I drive the truck.
There are a couple of benefits. The phone charges without having to dig down to the worthless charging pad and I get what I believe to be better quality music. I've invested in a sound system upgrade and I pay services with higher bit rate music, so might as well plug in to take advantage of it. My less frustration overall.
Yup, Android Auto does the same thing. It will be working perfectly then it finishes a track from any music app then the next track plays and no audio. Pause then play... nothing... pause then play again, sometimes that works sometimes not.
This has been an ongoing problem for me as well. Still not fixed.
I also toggle CarPlay on and off to fix it, but it's such a pain. I'm considering going with the "wired only connection" solution for now.
I'm a much happier person since I removed "wireless" car play from my life. I plug in every time and it's almost flawless. Wireless just isn't ready for prime time.