Just a busy signal is not so bad. But a fast busy signal is not good. This one scares me..
But the ultimate worse is when you get that annoying sound "tee too tooo" and a woman's voice saying...
"I'm sorry, but the number you are trying to reach is no longer in service. The line has been disconnected. Please hang up, or try your call again.. This is a recording."
____
Now that's when you should start sweating.