What does a fast busy signal mean on a cellphone?

A fast busy signal on a cellphone means that all of the cellular circuits are tied up by other users, and the call cannot go through to the intended person. In these instances, a caller must hang up and redial until the cellular circuits can transfer the call to the intended person.

This type of busy signal usually occurs during the aftermath of a natural disaster that creates exceedingly high call volume. Fast busy signals usually subside within hours of the unexpected event, when the cellular phone company restores the circuits or opens more circuits to meet the high-volume call demands.