As soon as I realise it's a telemarketer (I've had a lot who disguise themselves as other things), I'll flat-out interrupt to say, "thanks, but I'm not interested. Have a nice day!" and hang up, even if they're mid-sentence. The interruption evolved from too many having five-minute opening speeches about their companies. I'm not willing to waste their time or mine.
And for a lot of them, waiting to hear their response is seen as an 'opening'. I'll not wait to hear them say 'goodbye' back, because invariably they'll keep following their script and try to sell you stuff, or whatever they're trying to do. Quote a few will be starting to say, "but have you thought about how much you could save if-" as I press the 'end call' button.
Don't give an excuse, because a lot of companies call you back. It's like telling an amorous guy, "I'm married" instead of just "I'm not interested." The first gives the impression that if your spouse was out of the picture, you would consider him. Same for telemarketers: "I'm busy" means "if I wasn't busy I would listen and consider you" to a lot of people.