You can connect simultaneously to your car’s hands free Bluetooth and Highway’s BT music streaming (refer to your handsets BT connection guide).
In some cases there may be conflict with existing in car Bluetooth systems for iPhone users. You will notice Highway will play for a minute or so, then the car will take control, so the audio from Highway goes silent.
If you experience this some cars will let you change Bluetooth settings so the car is only used for hands free calling but not for music streaming. So you would need to use the car’s own Bluetooth just for phone calls and Highway 400 for Bluetooth music streaming.
If this setting is not available, to carry on using Highway you’ll need to tell your car to forget your iPhone.