While this is common sense thinking, I can’t help but think that Apple must have done their due diligence to design the mic on their iPhone to be more uni-directional and only pick up the sound from the zone that’s limited to the nearby area where the mouth is placed relative to the iPhone only, as to subdue the ambient noise as much as possible by only picking up the sound in the close proximity and not all the ambient sound all around.
The More mics (there are 2 of them on each hearing aid) are designed to be omni-directional most of the times to pick up everything, and only unidirectional when selected as such in a program. So if you’re in the default P1 which is usually omni-directional pickup, the unprocessed wide open field you hear from the More mics may be what your caller hear on their end, instead of the fully processed sound that you hear from your receiver.
But that’s just a guess and I don’t really know which sound the iOS protocol is designed to pick up, the unprocessed sound or the fully processed sound. But somehow, I think it’s more likely the unprocessed sound because if it were the processed sound, then a Mute in the ON app by you (to hear the streaming caller better without the ambient sound) would have rendered the same muted silenced on your caller’s end, which is not a desirable outcome anyway.
Bottom line is that there may actually be a difference if the iPhone mic were used as opposed to the HA mics. Not sure how one would be able to test this, short of simply ask the caller on the other end to tell you how they hear things as you switch between the 2 options while you’re in a very noisy place.