AT&T MERLIN LEGEND Release 4.0 Feature Reference page 146

Feature Reference
The sender is using extension programming or testing the telephone.
The sender with an MLX display telephone is using the Alarm Clock or
Directory feature.
The sender's telephone is forced idle for system programming or
centralized telephone programming.
The sender's telephone is not responding (for example, not connected).
The sender has activated Remote Call Forwarding.
A receiver is considered unavailable (his or her telephone does not ring) under
the following conditions:
The receiver has turned on Do Not Disturb (in this case, the sender can
call the receiver).
Another call is ringing or answered on the receiver's Cover button for that
sender.
The receiver is in extension programming or is testing the telephone.
The receiver with an MLX display telephone is using the Alarm Clock or
Directory feature.
The receiver's telephone is forced idle for system programming or
centralized telephone programming.
The receiver's telephone is not responding (for example, not connected).
Table 7 shows when a call goes to Group Coverage receivers, after first going
to any available Individual Coverage receivers (as described in Table 5).
If a call is sent to Group Coverage and no receiver is available, the system
continues trying to send the call every 5 seconds until a Group Coverage
receiver becomes available. This repeated attempt to send the call is call retry
timing . The 5-second retry timing interval cannot be changed.
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Coverage