Apple Watch takes over Bluetooth iPhone and I cannot make any calls

I have an Lexus 2016 RX350 and have used my iPhone 6s for two years with no issues. Recently purchased an Apple Watch (and both are up to date on software). If I wear the Apple watch while driving, it hijacks and ends the phone call. If I do not wear the Apple watch, there are no issues with using bluetooth to make phone calls. This has become super FRUSTRATING and I have no idea how to fix this issue except to NOT wear the Apple Watch (which of course makes no sense). Can anyone tell me how to fix this issue? Thanks in advance as I am one unhappy customer with Apple at this point.

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