If they ring asking to do market research tell them you work in MR. They can't ask you anything if you work in MR. Same if somebody approaches you in the street.
TPS is really good. We have it and nobody rings us asking if we want to buy a phone or if we need double glazing.
You can also get premium numbers blocked as well. Only problem is, if you like to vote for X Factor, Big Brother etc you can't cos the numbers are blocked.