Echo is a common issue with VoIP / Cloud calling. We usually find echo occurs when the microphone picks up what the speakers are playing, therefore causing a loop.
To fix this:
On a software phone or web software phone: Make sure you are using a headset or earbuds and microphone when making calls. Using a laptop with a built in microphone and speakers may cause echo.
On a hardware phone: There are settings called “Tx” and “Rx” gain in most phone models. By adjusting the Rx and Tx gain to be a lower number you will usually find the echo goes away.
Note: In some cases, just turning down the handset / speaker volume will fix this issue also.