TPS as stated. though I don't think there is a single case of them 'dealing with' the companies that don't bother checking their call lists against the TPS registered numbers list..
for those that persist two methods.
1) Say yes he/she in in the next room, I I'll just get him/her
don;t hang up - just put the handset in the drawer and leave it.
2) if you have the time and are evil - talk to them but take them off their script at every chance.
Both 1 and 2 get them upset.
if you know the deal behind the call that makes the money for them (e.g. combined gas and electric service) you can have loads of fun. had one of these and agreed the deal at the start of the call. by the end of it the bod in the call center was telling me that I wasn't going to take the service. I had never said 'No'. So I asked them to review the tape as I wanted the deal we had agreed at the start. Panic set in then as they still didn't have the information that would earn them their commission and I wanted that deal (yeah right). They hung up on me ! which is a no no for these telesales bods.
They are all used to being told where to get off so don't worry about being abrupt. no need to rude though.
A fave of mine is if you get stopped in the street by the clipboard brigade is listen to their "Do you mind answering a few questions" starter. and reply with
" By all means I will. But information today has a value and I charge ten pounds per answer. Now, what is your second question ?"
Have generated some pants wettingly funny physical responses from a few of them with this.
Addition:
being careful to adapt this so that not breaking any laws in this country - and made it equally convincing try summat like
http://www.bigducky.com/prank_calls/..._nightmare.htm