what could be the reason why my phone HTC one (M7) is not ringing out there's an incoming calls.

[Tee_chaze]

I have been using my phone for some time now HTC one(m7), these days I don't understand what's happening whenever I have calls coming in, it won't ring out but only vibrate. please any idea on how to rectify it will be highly appreciated.
THANKS.

 

[B. Diddy]

Welcome to Android Central! You've already checked the Settings>Sound menu to make sure the Ringtone volume is turned up, right? Does the phone play other sounds normally through the speaker?