[tsac]

If your phone does not have a hardware issue I would suggest doing a reload to factory OS and Apps. It is possible the OS is corrupted or one of the apps is causing this.
You can do a backup for any data you may want to keep but try the reload and testing before you reload a backup. If it works then you can load the apps one at a time to see if one is causing this.

If all fails then it may just be hardware and it might be worth selecting a ring tone that wont cause you any problems if it goes off.