It turns out my old phone didn't have the horsepower needed for these voice over ip apps. I know you didn't want to hear this but it was one of the reasons I moved up to newer models.
https://www.google.com/search?q=Samsung+Galaxy+S2&ie=utf-8&oe=utf-8#q=samsung+galaxy+s2+whatsapp+problems finds priors so try what others noted as "solved."
I own a pretty old Samsung Galaxy S2 loaded with Android 2.3.3.
I can use all the functionalities of Whatsapp but when I make or receive a Whatsapp call the other party cannot hear me. I can hear them perfectly.
Does anybody have a solution to my problem (other than ditching my S2 and upgrading to something better)????