In settings > Siri and search, are all three Siri settings enabled including “listen to Hey Siri”.
If your phone has IOS 15 or earlier – Settings > “Privacy & Security” > “Microphone” > make sure that “CarPlay” is enabled.
Make sure mic isn’t muted on the cars display in Carplay.
Wipe the lead terminals with a soft dry cloth and blow in lightning port on the phone.
Some of the above solutions crosses over, what is already working when you make a call, but it’s worth going through them anyway.
Also try using a different USB port in the car.
Try your phone in another car, preferably a MG
Maybe try a new lead for how much they cost on Amazon, because the MFI chips can start to breakdown.
What version phone have you got?