Fast Busy indicates the tones a caller would hear if the call could not be completed because of the lack of network resources. In NG9‑1‑1 the functional equivalent is “600 Busy Everywhere”
Fast Busy Tone, Reorder Tone
An audible tone of 120 interrupts per minute (IPM) returned to the calling party to indicate the call cannot be processed through the network.
Relevant NENA Documents