Iphone Xr Bluetooth Compatible with Mercedes C350?

It is possible the vehicle cannot handle more than one device at a time. Without the phone and Bluetooth turned on, go into the settings for the vehicle and remove the old pairing. Turn the vehicle off and then back on again. Go into the settings for the phone in the vehicle and select paring a new device, or however it is termed. Turn Bluetooth on for the iPhone to put it into discovery/pairing mode. See if the vehicle sees the phone and vice versa. This might help, If your iPhone, iPad, or iPod doesn't connect to or work in your car - Apple Support If the two still will not see each other, then contact your Mercedes dealer. The concierge should be able to assist you in pairing the devices.

What year is your C-class? I have my iPhone Xs paired just fine in my 2015 C-300. Did you have another iPhone paired in that car before?

You're welcome. Glad you got it sorted out.