Telephone systems, however, are also starting to use a related form of electronic mail: store-and-forward voice systems. The 3M Co., for example, has installed hardware at one location that enables employees to store a verbal message electronically when they can't reach someone else at 3M. Economical mass memories, such as magnetic bubbles [PS, Oct. '79], now permit voice messages, converted to digital signals, to be stored. Later, when the intended recipient checks his or her "mailbox," the call can be converted from digital signals back into speech "see "Machines that Talk...and Listen," PS, Aug.). A Pennsylvania AT&T subsidiary is testing a similar voice-mail system, and other hone companies and businesses are expected to enter this new market, too.