Hi, I have the same phone, same problem. I spent hours on Google and no one has a solution that I could find.
I just decided to contact LG directly through live chat, they told me the only solution to this problem is to factory reset your phone. I have yet to factory reset mine because of what a huge hassle this is going to be for such a stupid problem but I hope maybe it will help you guys since it appears no one is actually posting any answer to this problem.
I will get back to you all when I have reset my phone to see if it finally puts an end to this but it might be some time, not in the mood of reinstalling everything I had on the phone and backing up my contacts right now. If you happen to see it works for you, please post here so I can see myself and at least know it works for real.
It started happening on my phone ever since I hit "Quiet Mode" on accident, it never happened before then at all and I have had the phone for months now since last year, it has been the only problem I have ran into with this phone luckily. I guess I can't complain because it is leagues better than the old Optimus Elite I had, now that phone was a piece of crap and had so many problems, on top of the battery life sucking terribly.