MY S4 has no outgoing voice when calling on wifi. I hear the other party just fine. I travel the world for work, so the ability to use wifi is a requirement. It does not matter if I use the t-mobile wifi call, google voice or viber. Same problem on all. I've had this issue in hotels overseas and tested it at home, and tried it using the samsung provided headphones.
A t-mobile rep did a reset with the registration on foreign networks but that did not solve the issue. I think there is a software issue with the phone. Tmobile issued blackberries or i-phones had no problem. Neither did my previous phone.