There are at least 12 different varieties of telemarketing frauds, according to the American Marketing Association. It includes everything from saying “You've won a vacation, but you have to send us $1,000 first”, to insurance fraud, to simple high-pressure sales of real products. It really is highly diverse.
But the common denominator is generally targeting seniors—the primary target, because they're more trusting of people and are easily bullied. There are high-pressure sales tactics. Very common is advance-fee fraud. I'm sure we've all received an e-mail from someone claiming to be a relative of a deposed dictator in Africa who has $60 billion in a bank account and needs your bank account number. That's the predominant form of telemarketing fraud.