Echoes are caused by feedback between the phone speaker and the microphone. If your contacts say there is an echo while talking (they hear their own words), the problem is likely on your end.
We recommend reducing the microphone and speaker volume on your device. To do this, when the call is in progress:
tap the gear icon to open the microphone volume slider, and set it to the minimum;
reduce the speaker volume with the volume buttons.
If you are experiencing an echo issue on iOS, please contact Yolla support.