I have been researching these issues.
The 'Mailbox Subscribe Expires' is how often (in seconds) the ATA sends a voice mail status request (SIP subscribe message). The request doesn't immediately result in the ATA receiving current VM status, but places the ATA on the server's list of units to be notified when the next batch of VM notices is sent (at 5 minute intervals).
The ATA will send a SIP subscribe message on power up, and again every 'Mailbox Subscribe Expires' seconds. Voxalot will send an unsolicited voice mail status update notice (SIP notify message) whenever a new voice mail arrives.
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