This could be due to one of the following reasons:
1. If the Bluetooth connection is weak or unstable, it can cause audio delays or echoes, as the signal can not be transmitted clearly between the earbuds and your phone;
2. The other party’s phone can not be effectively canceling echo. You can ask them to try calling you from a different phone to see if your Redmi Buds 6 still have an echo;
3. The other parties have their speaker volume too high or are in an environment that causes echo, which can result in an echo on your Redmi Buds 6 as well.