Ensure that all unused BRI lines are disabled in the configuration. The Perle will always use the next available (enabled) channel for the Dial-out or Call back.
When viewing the Perle's event log note the channel that the Perle attempts to use for the Call back or Dial-out attempt.
Resource event log: S1CH03 S1M02 : freed from port 1
Resource event log: S1CH03 S1M02 : allocated to port 1 (CB)
Port: 1 Logoff user1
Resource event log: S1CH01 S1M01 : freed from port 1 (0-00010003)
Port: 1 Valid user logon user1
Resource event log: S1CH01 S1M01 : allocated to port 1 (DI-1234567890)
In the above example a call came in on Slot 1 Channel 1 and the Call back occured on Slot 1 Channel 3.
Channel 3 was enabled in the configuration but there was no physical ISDN line attached so the call failed.