A Pacific Bell ad
A Pacific Bell payphone
Pacific Bell was a telephone company operating in California, Earth in the late 20th century.
In 1986, the company published a register called Yellow Pages, which listed all businesses and government entities in a certain area. An advertisement for this was seen by Scotty, Sulu, and McCoy in San Francisco.
They also owned phone booths in the area. (Star Trek IV: The Voyage Home)
This is one of the few examples of product placement in the Star Trek franchise.