Callers hear an echo during calls?

Please try calling yourself, either to a landline in the house or to a friend's cell phone if they are with you.

Your phone has two microphones used for calls. There is a pinhole on the very bottom of the phone. That is the primary microphone. The pinhole on the top on the phone is the secondary microphone.

During your test call, speak into the primary mic at a normal volume. On the receiving end, check that the volume is "normal." Now put you S6 Edge+ in speakerphone mode and talk into the secondary mic at a normal volume. On the receiving end, check that the volune us normal. If you cannot hear anything try blowing directly into the pinholes. If talking into the mic produces no sound but blowing hard produces a tiny sound, the hole may be clogged with dust. If blowing produces zero sound, the mic is has probably gone bad. A failed secondary microphone can cause issues with background noise/ echoing.

Unfortunately if you confirm that your individual microphones perform nomally, but callers still complain of echoes, than you are out of luck. This would mean audio is being processed incorrectly, meaning a bad component in the motherboard.