Caller: Hello, can I speak to Mr [insert name of householder from telemarketers' list]?
Householder (wearily): Yes.
Caller: I'm Gavin from Acompanyyouhaveneverheardof Ltd. This is just a courtesy call....
'This is just a courtesy call' - six cheery little words that pop up from time to time, like a cold sore. In any reasonable list of phrases that could really spoil your day, 'this is just a courtesy call' will never challenge classics like 'I'm afraid we'll have to amuptate', but it's still a collection of words you'd rather not hear.
There are several options at this point. You can be polite, given that the human being on the other end is stuck a dead-end, high-stress job in a call centre and probably doesn't really want to make the call any more than you want to take it. Sadly, if you're polite and don't terminate the call at the earliest opportunity, the caller will persist (it's survival of the persistent in telesales) and will proceed to waste your time reciting the sales script.
You can be rude or sarcastic, but it's probably not fair on the person calling and it's not very satisfying anyway. Sadly, you're not speaking to the telesales managers and executives who created a business model entirely based on interrupting and annoying you in the middle of minding your own business at home - if you were, I'd recommend rudeness and sarcasm with relish.
You could pretend not to be the householder, or quietly put the handset down and walk away for five minutes until the caller eventually realises that they've been performing their sales script to nobody at all. You could make the caller repeat everything on the pretence that it's a bad line or that you're a bit simple, etc, etc, but these sort of alternatives are just variations on taunting the lowly sales drone who is no doubt already being punished enough by having to work in telesales just to make ends meet.
The best approach is to interrupt politely, but firmly, and let the caller know that you have registered with the Telephone Preference Service (TPS) because you don't want to be cold called. Beware of imitations, by the way. The Telephone Preference Service is free, but if you Google "Telephone Preference Service" at the moment, you may be directed to another service that calls itself the "Call Prevention Registry" (CPR) that claims to provide a better call-blocking service, but charges a fee. Although it doesn't cut out all cold calls, the TPS has reduced mine significantly and I have no reason to recommend the CPR
There's evidence that even the sort of people who advise telesales and call centre employers have realised that nobody believes the 'this is just a courtesy call' line any more. This isn't necessarily a welcome development. Although I'll be glad to see the back of this discredited phrase, it's only a matter of time before the marketers find another infuriating form of weasel words to keep potential suckers on the phone.