Shut Up and Listen
This is counter-intuitive because you're probably going to want to either interrupt the caller or just hang up. Don't do that. Believe it or not, you actually want to hear what they're saying.
During their opening monologue they will deliver three pieces of information that you want. They'll tell you the name of the company they're calling from and what they actually want and who they're actually trying to contact.
Why You Want To Know
It's unlikely that you'll care what they want as it's probably just to sell you something you don't want and wouldn't buy from a telemarketer even if you did, but since you're listening anyway you may as well find out.
Knowing who they're looking for is useful though as it may not be you. This information will determine how you eventually get rid of them. Knowing the company they're calling from is also useful as it helps you to make sure they don't call you again.
After allowing the caller to finish speaking, and asking them to repeat any of the information previously mentioned if you didn't catch it the first time, take one of the following three options.
What To Do
If you are their intended target and you are interested in what they want. Unlikely, but possible. Listen carefully to what they have to say and respond appropriately. Bear in mind that they're almost certainly reading a script off a computer screen and can't really go off on tangents, so stay focused and don't waste time. They're getting paid for this, you're not, so time-wasting really only hurts you. At the end of the call, if you have decided that you don't want them to call you again, tell them so. Be as clear as possible and make sure to confirm the name of the company. This makes it less likely that the dickhead you're speaking to will ignore the request.
If you are their intended target but you are not interested in what they want, politely inform them that you don't want to buy whatever they're selling, don't want to take part in their promotion, don't want to participate in their market research, or whatever it is they're trying to get you to do. If you have decided that you don't want them to call you again, tell them so. Be as clear as possible and make sure to confirm the name of the company. This makes it less likely that the dickhead you're speaking to will ignore the request.
If you are not their intended target, inform them of this fact. If you have decided that you don't want them to call you again, tell them so. Be as clear as possible and make sure to confirm the name of the company. This makes it less likely that the dickhead you're speaking to will ignore the request.
Once you have got to this stage, the caller should be attempting to get you off the phone, because either they got what they wanted or they will realise they're not going to. Let them read any legal disclaimers or thank you for your time or whatever. It's not like hanging up on them now will save you much time but it will probably annoy them, and they probably haven't put your number on their do not call list yet.
By following these simple steps you ensure that some people with shitty jobs have a slightly less unpleasant day and you waste less of your time answering pointless and annoying phone calls.
Some people simply hang up as soon as they realise it's a cold caller. While I can't fault this as a temporary solution, it is only temporary. You may not be called again today, this week or even this year, but your number will come up again and you will be annoyed again.
Some people feel they can't simply say "no" outright and will say they're too busy to take the call right now. This is pretty much the worst option you have, because the company calling you are more than happy to call you back later that day, the next day, the day after and the day after that. And they will.
Some people try to annoy or trick the caller. Remember, the person calling you is getting paid for this. You're not. And no matter how much of their time you waste, the company doesn't care. It's a drop in the ocean. You're not helping anyone, you're just wasting your time. You might be providing a call-centre worker with some entertainment though, so I guess there's that.
But by far the biggest mistake is in the way people deal with phone calls of all kinds. If your phone rings and it's not convenient to answer it right now, let it go. There are exceptions, but most most people (and you are almost certainly part of this group) are not in positions where it is vital that they be contactable at all times. If you're driving, don't answer the phone. If you're on the toilet, don't answer the phone. If you're in a place where it's too loud to hear someone on the phone, don't answer the phone.
If you think the call may be important, deal with whatever is occupying you and then call back. The world will not end if you let your phone ring for a bit.