I’m pretty sure @MDB is right, and when I’ll have time, I’ll look for the older thread about being trapped in phone calls with using HA mics in noisy environments with iPhones. There may be an option for Android, but an Internet search gives results claiming none for iOS. In iOS, the HAs are classic Bluetooth devices and are recognized as the headset profile (that’s the label that appears under the EAR icon in Control Center when I’m on a phone call or streaming). Headsets are supposed to have mics as well as earphones, so both input and output default to the HAs, and there is no way to change that in iOS. The only specific Hearing Devices settings are for MFi hearing aids under Settings, Accessibility, Hearing Devices. I’ve looked in Target, too, under Streaming Programs, and the only option is to control left and right HA mic volumes: (click to enlarge image)
The menu item for BT in the above screenshot only leads to a choice for using standard BT bandwidth or expanded bandwidth for better audio quality but possibly reduced stability in a phone call.
If one goes to iOS Bluetooth settings and clicks on the classic BT pairing (R-Phonak), one sees options similar to what @Emile030 showed for Android, but they don’t work.
If you click on Device Type, you can see a list of classic BT devices:
I’ve tried either “Speaker” or “Headphone” instead of Hearing Aid, but the HA mic still remains as the input device. iOS must be reading the physical type or usage protocol that my right Sphere is broadcasting and refuses to be hoodwinked by me changing the dropdown setting.
If I go back to the first screen (the initial R-Phonak BT settings screen) and choose DISCONNECT, that WORKS! (but it also disconnects the HAs from receiving the phone call, so I’m back to using the iPhone for both mic and speakers). 
Edit_Update: I also tested “Other” and “Car Stereo,” and they don’t disable the HA mic either.
I tested the R-Phonak Hearing Aid BT settings changes by using the iPhone Apple Voice Memo app. I started a recording, walked away talking to my HAs, and entered another room to see if the sound of my recorded voice on my iPhone diminished as I got farther away from the phone. My voice only dimmed when I disconnected my HA BT entirely.
The most interesting thing is that using the Car Stereo device option seemed to noticeably increase the HA mic audio quality. My recorded voice was much louder and clearer than for any of the other options. The Device Type settings screen does have a footnote that says “Specifying the type of device can ensure your Headphone Audio Level measurements are accurate.” So, I’ll test that setting in an actual phone call later tonight. Perhaps the Device Type settings choice influences the amount of signal amplification and noise filtering employed?