voice "falls" to speaker

[AC Question]

My phone is Xiaomi redmi note 2
In the last few days, I noticed a problem

When I’m talking (phone or buetooth car kit) after few seconds the sound is going to the speaker. The status signal still shows the original earphone/ bluetooh, but the voice is on the speaker.

After swiching to the speaker & back it’s OK, but meanwhile it’s annoying.

1- why is it so?
2- how do I fix it?

Other ideas?

Thanks