TPS anyone?
All our company numbers are corporate TPS registered, and I find that (at least when called by otherwise vaguely reputable UK companies) asking for the details of the caller in exactly the form and order required for the TPS complaint page (when necessary augmented by an explanation of why I'm doing this), generally results in a very rapid hang-up, and no more calls from them.
If all else fails, we have a special "procurement" number which we put them through to - a particularly painful "your call is important to us" message with stretched tape effects, big volume jumps etc.