I have the same problem on this phone. The busy signal comes when I plug in the headphone. Since this is not my primary phone, it doesn't need to connect to the cellular network all the time. One thing have been working for me is turning on the airplane mode and use Wi-Fi only.

I have the same problem on this phone. The busy signal comes when I plug in the headphone. Since this is not my primary phone, it doesn't need to connect to the cellular network all the time. About an hour ago, I turned on the airplane mode and use Wi-Fi only. The busy was signal magically gone. I will update the post if this method really works.

If it turns out that there is not an easy fix, I would go to the path of updating the OS or install an OS from another sources.