Hi linarthur
welcome to this Apple (user-to-user) community!
Re: "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?"
Seems the solution you seek would rely on whether your hearing-aids are "Made for iPhone" (Mfi) compatible or not. On the one hand, if Mfi compatible Apple's in-built software can help.
On the other hand, control of the hearing-aids must rely solely on Bluetooth
and firmware (and/ or an app) from the hearing-aid manufacturer.
You can check whether your hearing-aids are listed as Mfi compatible in Apple Support's list (link below):
(ie aids are listed alphabetically by manufacturer (brand) name)
List of Made for iPhone hearing devices - Apple Support
and
______
if listed : See: Use Made for iPhone hearing devices - Apple Support
Eg: says: "Phone or iPad: Go to Settings > Accessibility > Hearing Devices.
To see more settings, tap the name of your connected hearing device".
__________
if not listed:
Check with your hearing-aid manual, your hearing-specialist,
or the hearing-aid manufacturer (Eg: contact "support" or "help" via their website)
Please let us know if you need more assistance.
or
Apple's Accessibility Support team can often help with use of the accessibility software features
How to contact Apple for help with accessibility features - Apple Support
All the best :-)
