This has happened to me twice. Im going to be honest its not good from what I know. One day I made a call and when I hit send it made a beep I have never herd before. I was like "this sucks why is it doing that?". Well I never figured it out and the sound stayed but weird things started to go wrong with my phone within a week or so. Lockups,running way slow. It sucks but the only way I found was a hard reset to fix it and how it started was/is still a mystery. Then all was fine again for a month or so then I was making a call and beep. Oh no here we go again and just not to long ago I had to hard reset because of it.
There is a old post with some reg edits they said fixed it but I'm not believing it.
I'll see if I can find the link.
*edit* Old discussion right
HERE.
**Edit** Another thread about it
Need help with Mogul beeping.