Incoming calls only vibrate, phone is not ringing

[wibble]

The question was to find out whether normal audio playback through the speaker works before the phone call, as that's what's used to play the ringtone. If you can't hear audio from browser, music player etc. then that points to something affecting them all like mixer settings or a problem in the audio profile. If browser/music/whatever works but you have no ringtone then it's more likely a problem in the phone app or its configuration rather than a general audio problem.