According to Wikipedia, a Ring-back Tone (RBT) "is the audible ringing that is heard on the telephone line by the calling party after dialing and prior to the call being answered at the receiving end. This tone assures the calling party that a ringing signal is being sent on the called party's line, although the ring-back tone may be out of sync with the ringing signal. RBT is also known as Color Ringback Tone (CRBT)."
TelcoBridges and Ringback tones
TelcoBridges Tmedia and Tdev devices support the development of ringback tone solutions. They feature onboard IVR capabilities for audio playback as well as high capacity internal and external disk solutions for storing ringback tone files. Since it leverages an external application server, which can connect to essentially unlimited storage, the Tdev TMP6400 in particular supports the ability to record a subscriber's personal message and then use that as a ringback tone.
In November 2008, TelcoBridges announced that its partner Kingdom Telecom had successfully deployed a ringback tone solution with Henan Mobile, a subsidiary of China Mobile, with over 17 million subscribers. At the time of writing, the deployed system supported more than 4,000 E1 links.