The Bluetooth microphone doesn't work with any of the phones connected to the vehicle. It may be related to a navigation system problem I'm also having, or vice versa. I have a service appointment next week, so I will follow up afterwards.

Update from Jun 18, 2021: VW did an in-depth diagnosis (required by VWAG). The culprit was the vehicle Car Net module, which is essentially the vehicle's data hub. This caused the Bluetooth to become inoperative and GPS to be off by hundreds of miles. It also caused the remote start function to be intermittent. The dealership had to perform a series of tests due to the high cost of the bad module ($3,500), which thankfully was covered under the warranty. No issues have arisen since the repair.