Below mentioned can be few possible factors which can cause an echo in the line:
1) Network Bandwidth: Network Speed is one of the most probable factors that create an Echo in the line. Echo has been experienced if the there is a low network connectivity or the internet speed is not good. Please make sure that the quality Internet is being used.
2) When making calls from Laptop and using laptop's default speakers and mic instead of using headphones: It has been noticed that if the user is using laptop's default speakers and mic, it might be possible that the sounds from the speaker get captured by its mic and getting re-propagated to the user. Headphones are preferred for better call quality.
3) When boost is 100% (in Linux) or 36dB (in windows) in Microphone settings: This is also one of the reasons that can cause echo. In the Sound setting of the system, if the boost of the microphone is 100% (in Linux) or 36db (in windows), then we need to reduce it as much as possible to get rid of the echo. But do it till user get clear sound.
Steps to change the settings in windows system.
-In Windows Control Panel>Hardware & Sound settings>Sound>Recordings>Microphone>Levels>, reduced the Microphone Boost to 0.0dB.
-Control Panel>Hardware & Sound settings>Sound>Communication Tab, select 'Mute all other sounds" option.