In order to know exactly what happens with these announcements, we should know what system u are using. Generally, 1 announcement will represent for 1 or some errors in the system. In most case, “NETWORK BUSY” => there is not enough resource (such as TCH, SDCCH) for the calling subscriber to make a call; “OUT OF COVERAGE AREA” => the called subscriber is out of service (may be turned off or out of coverage area).
implicit detached = subscriber is out of service due to lost of radio signal or empty battery
explicit detached = subscriber turn off by power button on the phone.
Rgds