Car Bluetooth. Garbled voice on other end.

I service customer issues with their automobiles. Lately there has been numerous complaints of the person receiving a call from an Iphone that is paired to a vehicle's hands free Bluetooth, that the voice sounds "garbled" or fragmented. This has been most common with Iphone X. Even after the standard Bluetooth troubleshooting steps of unpairing, forgetting the devices and re-pairing there is no change. We have advised to try and turn their Apple watches to airplane mode; this has fixed a few cases. Even with the latest IOS version this is a very common issue. Is there a fix in development? In the mean time what other troubleshooting tips can we provide for owners experiencing this issue.