I've recently started amusing myself thus:
Given that, as I understand it, their priority is to keep you on the line and NOT hang up on you, I've hit on this to give myself something to do when this happens...
Scum-sucking parasite: Hello am I speaking to Mr. *****? (or other innocuous opening)
SSP: I understand you've been in an accident/entitled to PPI refund/need you to answer just three market research questions...
Me: What are you wearing?
... (the sharp ones hang up at this point)
SSP: What do you mean?
Me: (as creepily as possible) What sort of clothes do you have on?
... (most hang up by now, but so far just one hardy soul has ploughed on, off script and confused...)
SSP: Why do you want to know what I'm wearing?
Me: (Honestly surprised I've got this far and deciding OK, you asked...) "How am I supposed to masturbate to the sound of your voice if I can't picture you in my mind?"
... The invective I was subjected to before they hung up was in such heavily accented English (or possibly not in English) that I can't, unfortunately, report its contents. I honestly don't know what I would have done next, if they had, for instance, actually described their clothing.
Either way, it's given me a chuckle several times so far. I recommend it.
I can also report that it is AT LEAST as successful when deployed (by me, a man) against men as it is against women. And before anyone suggests this is in any way creepy or harassing-style behaviour, THEY rang ME.
If you're a woman, I'm going to stick my neck out and recommend you don't try this approach.