Depends on quality of offer and presentation. They are relatively cheap, so a low response rate is fine. Direct mail, or for mass markets, letter box drops can be respectful, informative and inviting introductions to new products. Highly objectionable to be interrupted and have to close the door on someone or usher them out. Wastes people's time.
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.