all trunks busy (ATB)

All Trunks Busy (ATB) indicates the state of a trunk group when all trunks, i.e., the main trunks of a local network or a telephone network, are active. In this case, the All Trunks Busy (ATB) service signal indicates that the trunk cannot accept another call. Intelligent Call Management( ICM) records the duration during which all trunks are busy.