Of course you have to know before you pick up the phone that it is a telemarketer – which can be difficult, even with call display. I worked in radio for years – when I answered the phone that way it was true.
Strangely enough, while I recall the odd solicitation on the office line, I don’t recall a telemarketer ever calling the on-air studio line at any of the stations I have worked at. Maybe they just know.
Try it today and see if it works for you. Don’t limit yourself to telemarketers – try it with your friends as well. After all, what else can you find to amuse yourself on a Wednesday?