Since I have had my Galaxy s5, I have had echo issues when using a speaker phone. I have tried three types of speaker phones -- Jabra, Jawbone and Logitech. Within five minutes, the person at the other end hears an echo. All speaker phones were new. I had no issues with my old Samsung phone (Vibrant).
I have had the phone replaced and I have the same problem. I love the phone except for this issue -- which is pretty big for me because I am on a lot of conference calls. I have to use the earphones instead and do not like being restricted with wires.