Why are my volume buttons on the phone not working when streaming Auracast^™ broadcast audio to my hearing aids?

For changing the volume of an Auracast™ broadcast stream, the buttons on the hearing aid can be used, as well as volume control in myPhonak app.

The volume of an Auracast™ boadcast stream is controlled at the transmitter and can not be adjusted via the volume buttons of your phone.

Why are my volume buttons on the phone not working when streaming Auracast™ broadcast audio to my hearing aids?