I don't know if they do this to heavy users or if it is widespread issue they are having.
it is all over the google forums.
Pick up the phone, dial a number and get fast busy signal. when this is happening all calls go to voicemail incoming.
9 out of 10 calls are busy now.
I see the issue dating back months now but it started for me in August 2012.
I found this thread while doing a search. I am having exactly the same problem described by Holysmoke. Is there any solution to this problem?