Here's an example of what I think is telemarketing done right. We have a client that's a roofing company. They scout areas that have seen a lot of damage from storms, note the houses with roofing damage, and look up their phone numbers. They call the residents, explain how their home insurance companies usually cover most of the cost of the roof repair, and offer to schedule a time to take a closer look at the roof and give an estimate.
I think it's mostly about targeting - don't just call tons of people with no reason, but focus on particular people who have a high likelihood of becoming a customer. Then call them with salespeople who know the industry, be helpful and courteous, and take people off your list who don't want to be called. And, of course, follow the law.
Unfortunately, there's so much bad telemarketing that the good stuff has gotten drowned out, so it's easy to see it all as bad.