What the title said.
It used to work fine, but at one point it simply stopped working.
Tested with two different earpieces, which were also tested with other phones and a laptop. Earpieces work fine with other devices.
When connected to my BV9700 pro, the sound is working fine over bluetooth, but the phone is still using its own microphone instead of using earpiece's microphone.

This makes earpieces effectively useless, and a rather shitty experience to use the phone itself considering its size and weight when it comes to even short calls, let alone long ones.