You can also try facebook audio calls, Facetime audio or Skype calls also.
They're all free.
But like you said above your wifi connection and speed will have to be good.
If either one of the parties has bad connection or slow upload or download speeds then the calls might suffer.
do you not use iOS FaceTime audio due to quality? i realize it requires both to have an iOS device, but its all that I use for overseas calls and its saved me thousands of dollars.
WhatsApp is annoying and poor quality.
Google Voice calls have never been that amazing in quality but I don't think it is their fault. Use any other service to make calls overseas and ... the call quality really sucks. I dated my then gf (now wife) in Peru for 4 years before we got married and it was impossible to understand each other over traditional international calls (also they were extremely expensive). So we resorted to Skype when we were on the computer and Hangouts / Facetime for when we were mobile - 5x better and free.
I used localphone.com to get a local number in Peru ($6/mo) she could call if she ever had an emergency (would ring my cellphone). We never had great success with international sms so we just used (then) Google Talk / MSN Messenger.
Even with terrible Peruvian internet, Skype calls were very acceptable quality. We use Hangouts to talk to family there now days.
