Never.
If you do get your prospect's attention and the response is negative, ask for forgiveness and a favor.
Yes, a favor.
Let them vent, first and all the way, the angrier the better, as long as they don't hang up or leave.
Why? Do yourself a favor: Try it until it works, then post a comment asking why.
Photo credit: Do me a favor and let me in… Beatlesgirl7