-trace with layers and filters
- - layer 1 and 2 state changes and messages
- - layer 3 isdn trace, process ids
- - messages between port, endpoint and call
- - port hunt and channel selection
- - dialing / routing
- - application process (action)
- - bchannel control (tones, dsp, filter, activation/deactivation)
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-avoid disconnect-collision (release if disconnect from both sides)