It is not rude to turn off your telephone by switching it on to an answering machine, which is cheaper and less disruptive than ripping it out of the wall. Those who are offended because they cannot always get through when they seek, at their own convenience, to barge in on people are suffering from a rude expectation.

Judith Martin (b. 1938) American author, journalist, etiquette expert [a.k.a. Miss Manners]

“Miss Manners,” syndicated column (1985-07-10)
    (Source)

Collected in Miss Manners' Guide for the Turn-of-the-Millennium, Part 2 "Home Life," "Communications at Home" (1989).