It is so much better if this incident sets a standard for their company and the telemarketers are forced to validate each time irrespective of their past association with the callers.
Other than random, unexpected cases of people trying to trick you, any customer in the world would, if they changed their mind about being interested, wait for the call back and say it then, not contact someone else at the company to be put on a do not call list.
I would hope that companies do more due diligence when a customer is not calling back. Maybe the customer changed their minds. The problem with telemarketers is that they don't care about what customers are thinking and so someone else went ahead and called him.
What I would prefer is (and this is where I think I don't want to give any inch of room for the sales person) is that they constantly have to be aware of what the customer's latest situation is. I would prefer all telemarketers get trained this way and not intrude. Don't you think this is better?