How to Perform Dial Tone Recovery in Exchange Server

Applies to: Exchange Server SE / 2019 / 2016 / 2013 / 2010

Dial tone recovery is an Exchange Server recovery method that gives users temporary mailboxes so they can continue sending and receiving email while the original mailbox database is being restored.

The original version of this article was written for Exchange Server 2010 with Windows Server Backup. Microsoft still documents dial tone recovery for Exchange Server Subscription Edition, Exchange Server 2019, and Exchange Server 2016. The recovery concept is the same, although the current procedure is based on Exchange Management Shell, a dial tone database, a Recovery Database, and mailbox restore requests.

In This Article

• 1. What Is Dial Tone Recovery?
• 2. Dial Tone Recovery Process
• 3. Same Server or Alternate Server
• 4. Recovery Database and Data Merge
• 5. Important Recovery Notes
• 6. Key Takeaways

1. What Is Dial Tone Recovery?

Dial tone recovery is designed for a mailbox database failure where restoring the original database will take time. Instead of keeping users offline during the restore, Exchange creates a temporary empty database and points the affected mailboxes to it.

Users can then send and receive new messages while the original mailbox data is recovered in parallel. After the failed database is restored, the recovered data and the temporary dial tone data are merged so users end up with both their old mailbox content and the messages created during the outage.

2. Dial Tone Recovery Process

Microsoft documents the process in four main stages:

1. Create an empty dial tone database. This provides temporary mailbox access while recovery continues.
2. Restore the failed mailbox database. Restore it from backup or recover it by another supported method.
3. Swap the dial tone and restored databases. Users return to the restored production database.
4. Merge the dial tone data. Use mailbox restore requests to merge messages created in the temporary database into the recovered mailbox.

The main Exchange Management Shell cmdlets used during the workflow include New-MailboxDatabase, Set-Mailbox, Mount-Database, Dismount-Database, and New-MailboxRestoreRequest. The exact command sequence depends on whether the recovery is performed on the original server or on another Exchange server.

3. Same Server or Alternate Server

Microsoft describes three dial tone recovery options:

• Same server: Create the dial tone database on the server that hosted the failed database. This is normally the simplest option when the server itself is still functional.
• Alternate server, then return: Create the dial tone database on another Exchange server, restore the original server, and later move the mailbox configuration back.
• Alternate server and stay there: Use another Exchange server for the dial tone database and keep the users on that server after recovery.

The alternate server must have enough resources to handle the additional users. Database portability also has version and database-schema requirements, so the recovery target must be compatible with the recovered database.

4. Recovery Database and Data Merge

A Recovery Database (RDB) is used to mount the restored mailbox database separately from the active production database. After the restored database is available, Exchange can merge mailbox data from the RDB into the final mailbox.

This is the same Recovery Database mechanism used for mailbox-level restore scenarios. If you need the detailed RDB workflow, see Restore an Exchange Server Mailbox to a Different Mailbox Using a Recovery Database.

During dial tone recovery, the final merge is especially important because the dial tone database contains the new messages created while the original database was unavailable.

5. Important Recovery Notes

• Preserve the failed database files before creating or mounting replacement databases.
• Confirm the restored database state and log requirements before mounting it as a Recovery Database.
• Do not confuse dial tone recovery with RecoverServer. RecoverServer rebuilds a lost Exchange server from configuration stored in Active Directory; dial tone recovery is focused on maintaining mailbox availability during database recovery.
• Validate mailbox access, message delivery, restored historical data, and the data created during the dial tone period before removing the temporary recovery objects.

6. Key Takeaways

• Dial tone recovery keeps users working while a failed mailbox database is restored.
• The temporary dial tone database contains new mail created during the recovery window.
• The restored database is normally brought back through a Recovery Database workflow.
• The final step is to merge the dial tone data with the restored mailbox data.
• The procedure remains documented for Exchange Server SE, 2019, and 2016.

References

• Microsoft Learn – Perform a dial tone recovery
• Microsoft Learn – Dial tone portability
• Microsoft Learn – Recovery databases