Enormous damage is done to a phone company's reputation. Wastes people's time and really upsets them. People hate junk faxes. Wastes a small amount of time, wastes fax paper and ties up the fax machine. It will be rather similar to receiving normal mail - but for plain text only and without the waste of paper.
Some people hate junk mail, but others find it an excellent source of product information.
One advantage is that you can often tell from the envelope what it is about and so prioritise the opening of mail to minimise wasted time. Unsolicited visits, phone calls and faxes are inherently disrespectful, so there are strong negative consequences. If direct mail material is presented in a respectful manner, then there will be little negative reaction. There is no defence. Answering machines are useless - telemarketers leave messages.