A Pager is a personal telecommunications device used to request a phone call from a pager subscriber and/or receive simple text communications in the form of e-mail and SMS. Pagers exist as one-way numeric and alphanumeric models that only receive incoming communications and two-way alphanumeric models capable of sending e-mails and SMS messaging on a specific frequency over a special network of radio base stations. The Pagers or Beepers are portable communication messenger devices in which the person sending the message uses a touch-tone phone and calls the pagers' number, then the person enters their number or voice message and within moments the pager carrier is notified by an audible beep or a silent vibration. The number or voice message can be read on the pagers' LCD screen. Numeric pagers are the simplest of the type of devices offering only a numeric display of the phone number to be called and pager codes. Alphanumeric Pagers are essentially modified versions of numeric pagers with sophisticated display to accommodate text. These devices would be given an e-mail address to receive text messages. Two-way Alphanumeric Pagers have the ability to send text messages typed in with a small keyboard. Until the popular adoption of mobile phones in the late 1990s, Pagers fulfilled the role of common personal and mobile communications. Pagers are still in use today in places where mobile phones typically cannot reach users and in places where the operation of radio transmitters contained in mobile phones is problematic or prohibited. One such type of location is a large hospital complex, where cellular coverage is often weak or nonexistent, and where radio transmitters are suggested to interfere with sensitive medical equipment. Pagers are also widely used in the IT world, especially in cases where on-call technicians cannot rely on more modern cellular telephone systems.