Why does a red phone with a line through it pop up when I use wifi to call out?

[AC Question]

I have selected use wifi, then cell to place a call. I have wifi in my home and have not had trouble like this. The dreaded red phone with a red line through it, meaning broadband is not available?

 

[Rukbat]

Does your carrier have wifi voice? Not all do. For those that do, you need voice-grade broadband. If your internet connection has enough phase jitter, it can't be used for voice, although data won't even know about the problem. (Try a site like Visualware to test your connection's quality for voice.)