Increasing microphone volume on iPhone 15 with hearing aids connected via Bluetooth

My hearing aids are connected to my iPhone 15 via Bluetooth, but with Bluetooth on nobody can hear me on calls! How can I increase my microphone volume and keep my hearing aids connected?

My iPhone streams calls directly to my hearing aids via Bluetooth - which is great because I can actually hear people on the phone! However, they can't hear me... I have to turn bluetooth off for them to be able to hear me which means I can't hear them in my hearing aids. Any advice would be most gratefully received.

[Re-Titled by Moderator]