The public switched telephone network (PSTN) in the United States operated as a virtual monopoly from 1877 until the government-sanctioned breakup of American Telephone & Telegraph (AT&T) in 1984. Since that time, deregulation and technological advances have given rise to an array of competing wired and wireless telephone technologies. The most significant factor driving these changes has been the shift from analog to digital technologies.

On a March evening in 1876, Alexander Graham Bell learned in his Boston laboratory that his idea for conveying sounds through a wire worked. Bell's crude system harnessed the acoustical energy of speech by using sound waves to vibrate a thin diaphragm attached to an electrically charged wire that was dipped into an acidic solution. As the vibrations caused the depth of the wire to vary, the electrical resistance of the wire varied in proportion to the wire's depth. At...