Cannot adjust speaker volume on Bluetooth headset

Using Zoom on Windows 11 with all updates installed. I am hearing impaired and my Phonak hearing aids function as a Bluetooth headset. When I am in a Zoom meeting I cannot adjust the output volume using using the volume slider in Windows or using the volume slider in Zoom audio settings. Both the input and output sliders in Zoom audio settings have an X next to the speaker icon on the left end of the slider indicating, I assume, that the sliders are disabled. My Phonak hearing aids are selected as both the input and output device in Zoom. What must I change to be able to control the output volume? Thanks.