Just "upgraded" from a Moto X to a Samsung S5--the only reason being is because the camera quality of the MotoX was a little subpar in my opinion. My husband son both have S4s and LOVE their phones. Sadly, I had dropped by MotoX and the screen had cracked beyond repair/hope so I used my early upgrade to get an S5.
Everything on the S5 works EXCEPT the PHONE!! Since I got the phone (2 weeks ago) I have nothing but complaints from every person I have called. My doctor's office actually hung up on me because they couldn't understand me. Generally, the problem seems to be with an "echo back" for the recipient of the call. From my end, it all sounds fine but they can't understand anything between the echoing and the garbled sound. I have tried to trouble shoot this issue and have tried clearing phone cache, upgrading to HD voice, changing my in call settings, holding the phone differently, taking off the phone privacy settings, and cleaning the MIC. NOTHING has helped.
I didn't read any reviews on the internet prior to my upgrade but now that I have I can't decide if I just have a lemon or if the S5s are lost cause. At least with Moto X the voice worked great and I could make calls with my Phone...LOL imagine that!! Just wondering if anyone else has anything to add to try or in your experience if I am better off just trying a different make/model. Please don't tell me I need to explore Apple. 
Thanks in advance for all ideas!