You could try the free app Talkatone. It connects with your Google voice account and allows you to make calls to other mobile devices or landlines. It's free to call within the USA.
I've used it on both WiFi and 3G and it works pretty well. I had the Apple ear buds (the ones with the microphone on the cord) and I was able to both hear and be heard just fine. Worked really well when I had to call my brother-in-law, and a garage, when I had car trouble...
Marilyn