In many cases echo is a symptom of acoustic feedback from the phone of the party you are talking to. Your voice travels across the phone system to the other party, their phone's speaker plays the sound, then their phone microphone picks up on that sound and it is transmitted back to your phone as an echo.
Speaker phones are notorious for experiencing this problem due to their loud speaker and sensitive microphone. If you experience echo, try asking the other party to turn the volume down on their phone. If they are using a speaker phone see if lowering the speaker volume or picking up the handset eliminated the echo. Some phones are more prone to creating echo issues. If it is possible have the other party try using a different telephone. If the other party is experiencing the echo, repeat these steps on your system.
If these troubleshooting steps do not work you can contact OnSIP Customer Success for further help.