You can ask Swisscom Directories (the company currently maintaining phone/fax number directories for historical monopolistic reasons, alas this might change) to mark your entries as Does not wish to be pestered with ads.
Within a directory entry this is a simple asterisk near the number(s) and includes (and is usually respected) direct mail.
While this is not universally respected by telemarketers (it mostly is, though) it's a great weapon to field unwanted calls fast and (in most cases) fairly politely. A sample dialogue can look like:
RING, RING...
Me: Hello
TM: Hello, who is in charge for office supply purchases?
Me: I'm sorry sir, this number is marked as not wishing to receive telemarketer calls. I'm not interested in your offer
TM: (usually) OK, bye bye
This is not absolutely consistent, but works almost always on (rare) calls. Somethimes you do have persistant bastards like the guy calling three times in a week (the third call was not handled politely) or idiots that have the gall to ask why I'm not interested in their offer, which is really non of ther fscking business.
In this case I switch very rapidly to unpolite mode and with real persistant basatrds I can get outright obnoxious.
It's very simple: It's my phone, it's my time and it's my right to chose if I want to talk to you. I don't owe anybody an explanation why I want to be left alone. Period!