Iphone 11 and VW MiB3 link question.

Car is a 2022 T-Roc R with MiB3 infotainment unit. UK.

We connected Waze (iPhone 11) to the satnav but didn't really like it so went back to using the VW satnav and Waze hand held (wife/passenger) for the traffic updates etc. All works well but when we plug in the usb power lead from the car socket the iPhone looses all volume and Waze voice become an almost silent whisper. I have got something wrong but no idea what ! There must still be some 'link' between the phone and car that I need to disable but I have no idea what or where to look !

Any ideas/advice would be appreciated.

Many thanks. Brian.