The echo occurs when the person you're calling turns on the loud speaker. In that case, the delay of transmission causes your voice in the other line to be heard by their mic and transmitted back to you.
If that's not the case, maybe it's because of a faulty connection or broken microphone.
The person on other end of the call is answering you while the phone is on loudspeaker. Or it could be also that he or she is in the bathroom, toilet or an empty room. There are the major reasons why one hears echo when answering calls. It's not a speaker problem or anything being wrong with your phone.
When your talking to someone through the phone you might hear some annoying echoes can happen for one reason ,The microphone located in the mouth piece inside your phone can decide to skip backwards into the ear piece of the phone which causes your callers to hear echoes.
The echo that you are hearing might be caused by "sidetone" delay in your conversation.
Sidetone is a process between your phone and the network that allows you to hear your voice when talking to somebody on the receiving end.
1. Remove yourself from your immediate environment and find a better signal reception.
2. Kill or close apps you don't need open.
3. Call the person on the receiving end to establish a new connection.
4. On some phones, turn echo cancellation to on.