AT&T MERLIN LEGEND Communications System Release 2.0 Maintenance And Troubleshooting Manual page 163

Central Office Problems
Automatic Testing for Trunks
The automatic maintenance program takes the ports out of service when a
trunk is malfunctioning. When a port is out of service, it is maintenance-busy.
The trunk is periodically tested to put it back in service whenever possible.
Maintenance-Busy Status
When maintenance-busy is enabled and a trunk cannot be seized for an
outgoing call, the trunk is automatically put into a maintenance-busy state.
When a trunk is in a maintenance-busy state, no outgoing calls can be placed
on it. However, the trunk still receives incoming calls.
NOTE:
Maintenance-busy does not apply to DID trunks because DID trunks
cannot be used to make outgoing calls.
While the trunk is in a maintenance-busy state, the software periodically runs
tests attempting to seize the trunk:
if the seizure is unsuccessful, the trunk stays in a maintenance-busy
state
if the seizure is successful, the trunk is placed back in service
if a test is running when an incoming call is received on the trunk, the
test is dropped and the call is processed
No more than 50 percent of trunks in a pool can be maintenance-busy at one
time. The only exceptions are as follows:
user-imposed maintenance-busy (for example, module replacement)
module maintenance-busy (for example, a loss-of-service alarm in the
100D module)
no external release at the central office end of the line
Trunk Errors 5-5