Soooo, currently experiencing the same thing and sadly I believe that is hardware. :-( BUT, I pray that it's software and good ol" Sammy is going roll us all out a gift really soon. However, in the meantime I have turned on my
'Hearing Aid Setting' ( Settings>Accessibility>Hearing>Hearing ***** *on*) to see if that helps. IDK, but I'll post back in a week with the results.
Just got an OTA update on Vodafone Ireland to software ending ARD4/Baseband ARD7 so it looks like the wider push has started
About 167MB and takes 5 minutes or so to install. Will test it tomorrow but hopefully this will fix the call I problems