Phone Does not Ring Even Though Ringer Volume Settings All At Set to Maximum

Only today I have noticed that when someone calls me the phone does not ring nor does it vibrate.
 
All Ringer volume settings are set to maximum volume and vibrate in on but still the phone does not ring.
 
I have noticed that the moment an incoming call comes in, the mute icon (speaker icon with x in front of it) flashes on for a fraction of a second before disappearing again and the phone vibrates for a fraction of a second as well. It is as if all the settings are correct but the second a call comes in something launches and switches the volume to mute and switches off vibrate but not quickly enough to prevent a fraction of a second of vibrate and the mute icon appearing for only a fraction of a second as well.
 
I have tried a system restore from by Titanium Backup but the problem still exists. Can anyone help please?

Same problem here, and it's driving me NUTS!!! It's totally random, although lately it seems like it's getting even worse. At first it was just one friend who called me, and no matter what my volume settings were, it was zero volume, or in some cases like 1% volume... like it was my phone ringing off in a distant room somewhere. Now it's happening with multiple callers. I've done a hard reset, I've tried the whole 'plug and unplug' the headset, then adjust the volume in the "music" app... none of it worked. I don't even normally use my wired headset, I use a moto H730 bluetooth. No, it's not causing any problem either because it'll do this even when the bluetooth is off and unpaired. I now have to keep my phone within my peripheral vision at all times, or I'll miss calls all day long. If I see the screen come on, that means I have an incoming call. If I don't solve this problem soon I'm going to self destruct, and I'm taking this phone out with me. This must be resolved, it's affecting my day to day business now.

Otherwise, I love it.
SGH-T989 "Titanium" Series, T-Mobile, Android 2.3.6, Kernel 2.6.35.11