Like you, I get 2 or 3 calls a week some weeks, and then maybe nothing for a week in between. If only it was as simple as saying "I'm not interested, but thank you" and then hanging up, this wouldn't even be discussed here. Whilst I respect these people are only doing there jobs, they are extremely tenacious. For the first 3 minutes or so, you don't even have an opportunity to speak. Typically, the call begins with "am I speaking to robd?" Answer - "yes". "How are you today?" Answer - "Good thanks" and then off they go. When they eventually give you an opportunity to speak and you say that you're not interested, they just keep going.
I had one bloke, calling about website development expertise, from India I think, who just kept on, and on, and on and then became so aggressive toward me because I wouldn't meet with his people for 5 minutes, he actually abused me and then hung up on me.
Unfortunately, I am not in a position to be able to hear the typical overseas delay and just hang up, because we trade with several suppliers from overseas.
Recently, I tried the play-along with a "Telstra" business supplier who, as they all do, said we could save $$$ every month on our landline call rates and line rentals. I let them talk for a good 15 mins saying "that's great, blah, blah, blah". When the operator transferred me to the supervisor to sign us up on the great new rates, I ask him to confirm that they were going to pay out our current Telstra contract which is circa $20k (being phone system contract with at least 2 years left to run). The quickest backtrack ever.
The printer cartridge people are almost as bad too.