This is happening to me all the time. And no need to reboot and like in my case reinstalling new OS which anyway wont work.
This is a stupid bug which had created a lot of panic in my case.
Ok so if it is the same bug as me, you should see the icon for headphones when you click speaker phone. If it does it means you answered a call while listening to music and having the headphones plugged in and you probably unplugged it during your phone call too.
So the fix is to call any number (voice mail for instance) and plug back your headphones in and while they are activated switch back to head set with the button. If it doesn't work, try other approach like plug in and out the head set during a call.
Hope that helps
**I just noticed you have a bold but try it anyway, sounds too familiar to be that different
