I have M70-R. Do you get phone sounds – clicks and taps – in your ear over the BT streaming audio? This could be why it keeps changing to the streaming program.
You can either turn off the phone sounds in the phone settings (assuming iPhone has such a thing; I have Android). Or you can turn off automatic audio streaming in the BT settings. You could still make/receive phone calls with the Marvels but it wouldn’t keep switching over.
Mine switches over whenever I do something on the phone unless I deactivate audio streaming in the BT settings. If I want to play audio, I can pop into the settings and re-activate it. This is what the settings window looks like on Android.