Had a pretty serious issue starting on Saturday and my troubleshooting seems to point to WiFi calling on my iPhone. It started when I was trying to call my Wife who also has an iPhone 6 I kept getting a fast busy signal or going straight to voicemail. Calling other numbers was generally successful but would sometimes get a fast busy also. Folks calling me would sometimes get a busy signal or go to voicemail. Other services such as texts and internet seemed to work fine. These issues were happening all over town and not just on wifi. I turned the WiFi toggle off and still had issues. Finally logged onto my Sprint acct and removed the WiFi calling provision from both of our lines. No issues since. So if anybody with an iPhone on Sprint is experiencing call issues you might want to try removing the feature from your acct before taking more drastic measures. Both Apple and Sprint support wanted me to wipe and restore. Several people on S4GRU are also reporting the same issue. Also just to note this seems to only be impacting iPhones. Other phones on Sprint capable of WiFi calling don't seem to be having issues.