A ringback tone is a noise, song or short audio clip that plays over the speakers of the receiving line when somebody calls a phone. The ringback tone continues to sound until the phone is answered or the call goes to voice mail.
The most common ringback tone is a simple trilling noise. It indicates that a call has gone through and that there is an active phone line on the other end. Some mobile companies allow customers to customize the ringback tone that plays when somebody calls their private lines. Unlike ring tones, ringback tones cannot be downloaded. Ringback tones are hosted by the network carrier that provides service to the phone on the receiving end of the line.