The sound made by the phone network to indicate a ringing phone on the other end of the line.
In other words, the sound you hear when you call someone else; and the sound someone else hears when they call you (before the call is answered.)
Some phone companies allow you to customize the ringback tone others hear when they call you. The standard "ring ring" can be replaced with a clip of music or a voice recording. This is a feature of the network, not the phone.