Q: iPhone 5 microphone not working when speaker on

I've got a strange situation with my iPhone 5.

When making a FaceTime call, the iPhone goes on loudspeaker. The other party can't hear me. The mic is not set to off (checked).

When making a telephone call, with the iPhone on loudspeaker. The other party can't hear me. The mic is not set to off (checked).

When making a telephone call, with the iPhone not on loudspeaker. The other party can hear me, everything is fine.

When using memo function. A memo can be recorded and the play back souds fine.

When making a telephone call, with the iPhone  not on loudspeaker. The other party can hear me, everything is fine. When during the call the phone is switched to speaker, than the other party can't hear me.

My analysis:

As the mic is working with a telephone call and memo, the mic (downside of phone), is not broken.

Question:

Why is the mic not working while the phone is on loudspeaker and working fine when the iPhone is not on loudspeaker?

Issue was present before iOS 9.3.2. and still after upgrade to 9.3.2