I used Fring for a long time, but then am all in for FaceTime audio. Its the easiest to use, no setup, just works. But thats just for personal. I've been using Skype for work, with a $2.99 unlimited US calling plan, and a workaround is having my Google Voice number as the caller ID.
Complicated but outgoing calls are through Skype (with GV number as caller ID) and incoming calls to my GV number come to my phone, using cell minutes. Text is done through Google Voice. I called up my carrier's customer service asking about second line/second number and it was a joke.
So now I'm checking out Line2. Call quality is really good and even better on wifi. Its $9.99 a month after the trial. Good set of tools, looks worth a shot. Best part about VoIP apps....no contracts!