Turn off the Google app TalkBack, the icon is a big HAND orange and white colors. This app is causing an echo when you use your Galaxy S5. Problem was, you can't hear the echo yourself, but the person you talk to will complain about an echo. I would get the echo when I went to speakerphone, but other owners may have this issue on regular speaker too, I don't know, but just turn off the TalkBack app, it's interfering.
Message to Verizon and Google, YOUR CUSTOMERS ARE TIRED OF BUYING PHONES WITH LOADED DOWN GARBAGE APPS. What is this? A PC? Time is money in this world and your customers should not have to waste their time working the bugs out of a $500 dollar phone.
Yesterday I was on a call and the person on the other end told me to call again because of an echo. I hung up and closed all of the open applications on my phone and when I called back there was no echo. It could be a coincidence, but it is worth trying.
I figured it's kind of like using Skype on a computer with 15 programs open. It can cause delay and sound/video distortion if the computer's memory is being used up by other programs.