Some time ago, I discovered ten words that work nicely to make persons hang up when they call to sell me warranties, lower my interest rates, or warn me that my social security number is being used from fraud, etc.: "Is there a number I can call you back at?" Some years ago, I found that "What company do you represent, and in what state is it incorporated?" shortened calls nicely.
In something like five years of asking for a call-back number, I think that I've hit one naive fellow who actually did give me his employer's number. Unfortunately, I lost it before I could find someone to complain to.