Xiaomi 4 can't hear the voice on the phone. What happened?

This is a system problem of Xiaomi mobile phone, and the general solution is to restart the mobile phone to solve the problem. The following are specific ways to deal with this problem:

1. If the mobile phone is still under warranty, then go directly to Xiaomi for system maintenance.

The temporary solution is to restart the phone, and then you can resume normal calls.

3. After restarting the mobile phone, connect to WiFi, and then upgrade the system of the mobile phone.