Most 20th century answering machines used magnetic recording, which Valdemar Poulsen invented in 1898. The creation of the first practical automatic answering device for telephones, however, is in dispute. Starting in 1930, Clarence Hickman worked for Bell Laboratories, where he developed methods for magnetic recording and worked on the recognition of speech patterns and electromechanical switching systems. In 1934, he developed a tape-based answering machine which phone company AT&T, as the owner of Bell Laboratories, kept under wraps for years for fear that an answering machine would result in fewer telephone calls. Many claim the answering machine was invented by William Muller in 1935, but it may already have been created in 1931 by William Schergens whose device used phonographic cylinders. Ludwig Blattner promoted a telephone answering machine in 1929 based on his Blattnerphone magnetic recording technology. In 1935 inventor Benjamin Thornton developed a machine to record voice messages from the caller. The device reportedly also was able to keep track of the time the recordings were made. Although many sources maintain that he invented it in 1935, Thornton had actually filed a patent in 1930 (Number 1831331) for this machine, which utilized a phonographic record as the recording medium.