Hi Guppy.
- Set up a google voice number (free)
- download Talkatone app and go through prompted set-up (free - and intuitive)
- Play with sound settings till you get a clear signal
Talkatone is paid for by ads at the bottom of the screen. You can kill those with a small fee if you wish ($1 a month I think). I did the first couple months but don't care enough to do so now. A nice addition to this set-up (which I use) is a prepaid t-mobile sim card installed (iphone has to be unlocked; can do via AT&T customer service or jail-breaking). Most the time I have wifi and don't need the sim card, but it's nice to have when I don't have wifi.. just have to turn off the airplane mode and I'm good to make cellular calls. It also provided me with a secondary number for the cell service (I had ported my number to google voice so I could have my primary number as the number used for VOIP). Most of all, the $20 spent on the prepaid has lasted me for three months (compared to 3x $80), and I get better reception in my office using the voip than anyone else does using cellular data.
Hope this helps; let me know if you have questions/hiccups.