Phone Enters Silent Mode Automatically When It Rings

[zixin]

It seems that most times when people call me the phone won't ring. It actually goes into silent mode (verified by checking the sound selection). As soon as I am done with the call and hangs up or if the call goes to voicemail, the phone goes back to normal mode (again verified by checking the sound selection). When I power cycle the phone, it sometimes seem to be able to ring once and then the problem resurfaces. I have no idea what is wrong with the phone.