why is my iphone 6 only vibrating not ringing out
When I have read the blog this seems to be a common problem which has not been solved yet, as they state switch phone off and then on again, it should help for awhile!
settings for 'sound volume' and 'sound on/off' and of 'do not disturb' and so on , correctly set?
This may help: press and hold the HomeButton and the PowerButton at the same time and hold (about 10 secs) until you see the white apple logo appear, then let go and let the phone startup.