iPhone X/8 and iPhone 6,7 users are facing issue while using carplay app to make voice call while seating in their car. When other person calls and iPhone 7 is connected via CarPlay to car, person on other hand hears voice distorted/ blurred which makes it hard or impossible for them to understand. On the other side when iPhone is connected via Bluetooth there are no such issue seen and the call works just perfect.
When contacted the Apple customer care sometimes the response is received that its issue with car manufactures, which is not at all convincing as the same car system works fine with earlier version of iOS and no such issue in voice call with carplay was reported. It's certainly an APPLE issue, As after upgrade to iOS 11.3 update and all of CarPlay calls are garbled. CarPlay works fine all but voice calls out. Sounds like broken speech. The issue is seen across all car like Beetle, BMW, Jetta etc. Even upgrade to latest iOS 11.3.1 does not fix the issue.
To fix the issue if you try to reset the network, or reset your iPhone, does not seems to fix the issue. However there are certain reports that this has been verified as an iOS 11.3 problem by plugging in multiple pre iOS 11.3 phones into the same car using the same lightning cable.
How to fix iPhone carPlay voice muffle to poor quality with iOS 11.3:
Honestly speaking there are no working fix for the issue as it is a bug in iOS system and resetting/enabling-disabling setting will not help to fix the issue. However you can avoid the issue by following:
Fix 1: Restore the iPhone to older version
A simple solution to this issue is to restored iphone back to the iOS version where (eg 11.2.6) where you have never seen this issue.
Fix 2: Upgrade to latest iOS releases
Some users report that after upgrading to iOS 11.4 beta the calls seems to better quality of voice on carPlay but the issue is not completely fixed. So as soon as the iOS 11.4 is released it is recommended that to upgrade the iPhone to it as Apple may fix it in that.