What is telephone echo?

Echo is a headache and frustration that often invades telephone communications. Echo is the phenomenon wherein speakers hear their own voice echoed back after a brief delay.  Although some technologies mitigate many causes, echo is still a problem even with the advent of VoIP. Sidetone is a unique kind of echo where sound effectively loops back between the speaker and microphone at one end of a phone call.   

Every telephone call has some echo, the question is whether that echo is apparent to the callers.