Verizon is the culprit, something in the OS. My Galaxy S5 on the latest firmware offered by VZW had a similar problem. When I click on speaker on my phone it doesn't switch to the speaker microphone. It's been doing this on my phone since day 1 and it's only on Verizon that I've noticed the problem. Friends with the AT&T variant have none of the problems as me, even though I'm sure there are some on it too.
Posted via the Android Central App