Typically, solid-state oscillators have replaced them. Originally this voltage was used to trigger an electromagnet to ring a bell installed inside the telephone, or in a near-by mounted ringer box.
Not literally a tone nor an actual (bell-like) ring any more, the term is most often used today to refer to customizable sounds used on mobile phones.
Mobile phones have been fully digital since the early 1990s second-generation ("2G") devices, 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.
Some novelty phones have a ringer to match, such as a duck that quacks or a car that honks its horn.
The first commercial mobile phone with customizable ring tones was the Japanese NTT Do Co Mo Digital Mova N103 Hyper by NEC, released in May 1996. In September 1996, IDO, the current au, sold Digital Minimo D319 by Denso.
A ringing signal is an electric telephony signal that causes a telephone to alert the user to an incoming call.