However, later charges, on top of telephone line charges were levied. Legacy terminals communicate over standard telephone line or Ethernet connections. DTMF was initially developed for telephone line signaling. The Musical Telegraph used steel reeds oscillated by electromagnets and transmitted over a telephone line. Sky also have their own service accessible from dialling 123 from a Sky telephone line. An LC filter was required to prevent telegraph clicks being heard on the telephone line. Several different telephone line modulation techniques are used by fax machines. Kwangmyong has 24-hour unlimited access by dial-up telephone line. Virgin Media is the second biggest player in the residential telephone line market. Some radio users have inappropriately started using the Circuit Merit telephone line quality measurement. Most NIDs also include "circuit protectors", which are surge protectors for a telephone line. Hz to indicate an unused telephone line. The vocal was piped in through a telephone line. Dial-up connections generally require the dedicated use of a telephone line. This includes the 6P2C used for telephone line connections and 4P4C used for handset connectors.