its a protocol issue from your belkin receiver. it just doesnt know how to handle bluetooth phone calls. youre just thinking about it the wrong way. you want your phone to change the way it acts, when in actuality you need to teach the belkin receiver. also you would need a microphone for your belkin receiver.
the phone works the way it should, it handles bluetooth calls and bluetooth music. but the two items are not, and cannot be the same.
think of this, your listening to music and the phone rings. the music stops because something different is happening. how could the phone differentiate between the two if they were handled the same way?
we have a work vehicle that is the exact opposite of your belkin receiver, it handles phone calls but not music. i have a car system that does both. what you need to do is upgrade your belkin receiver. maybe there are add ons for the receiver.