A ringtone or ring tone is the sound made by a telephone to indicate an incoming call or text message. Not literally a tone nor an actual (bell-like) ring anymore, the term is most often used today to refer to customizable sounds used on mobile phones. A phone rings when its network indicates an incoming call and the phone thus alerts the user. For landline telephones, the call signal can be an electric current generated by the switch or exchange to which the telephone is connected. For mobile phones, the network sends the phone a message indicating an incoming call. A telephone ring is the sound generated when there is an incoming telephone call. The term originated from the fact that telephones originally had a ringing mechanism consisting of bells and an electromagnetically-driven clapper, producing a ringing sound. The aforementioned electrical signal powered the electromagnets which would rapidly move and release the clapper, striking the bells. This electromagnetic bell system is still in widespread use. The ringing signal sent to a customer's telephone is 90 volts AC at a frequency of 20 hertz in North America. In Europe it is around 60-90 volts AC at a frequency of 25 hertz. Some non-Bell system party lines in the US used multiple frequencies (20/30/40 Hz, 22/33/44 Hz, etc.) to allow "selective" ringing. In Australia the ring signal averages 100 V AC at 25 Hz.[1] While the sound produced is still called a ring, more-recently manufactured telephones electronically produce a warbling, chirping, or other sound. Variation of the ring signal can be used to indicate characteristics of incoming calls (for example, rings with a shorter interval between them might be used to signal a call from a given number). A ringing signal is an electric telephony signal that causes a telephone to alert the user to an incoming call. On a POTS interface, this signal is created by superimposing ringing voltage [90 volts AC at 20 Hz in the USA] atop the -48VDC already on the line. This is done at the Central Office, or a neighborhood multiplexer called a "SLC" for Subscriber Line Carrier. (SLC is a trademark of Alcatel-Lucent, but is a term often used generically.] This ring voltage came from various sources. In large Central Offices, there were 48VDC motor-driven generator sets for both ringing & other signals such as dial tone and busy signals. In smaller offices, a special Sub-Cycle magnetic oscillator was used. More recently, solid-state oscillators have replaced them. In old phones, this voltage was used to trigger a high-impedance electromagnet to ring a bell on the phone. Fixed phones of the late 20th century and later detect this ringing current voltage and trigger a warbling tone electronically. Mobile phones are fully digital, hence are signaled to ring as part of the protocol they use to communicate with the cell base stations. In POTS switching systems, ringing is said to be "tripped" when the impedance of the line reduces to about 600 ohms when the telephone handset is lifted off the switch-hook. This signals that the telephone call has been answered, and the telephone exchange immediately removes the ringing signal from the line and connects the call. This is the source of the name of the problem called "ring-trip" or "pre-trip", which occurs when the ringing signal on the line encounters excessively low resistance between the conductors, which trips the ring before the subscriber's actual telephone has a chance to ring (for more than a very short time); this is common with wet connections and improperly installed lines.