Just came across an excellent post by Dale Hardin on Dial Tone Database Recovery Process in Exchange 2007.

Just to clarify what a dial tone database is, this is a blank database you create to allow your users to send and receive new e-mails while a failed database is being recovered.  To create it, you simply move or rename the original database file (with a .edb extension) and then mount the database from EMC or EMS.  When the Information Store fails to find the .edb file in the specified location, you will be prompted to create a blank database.  If you agree, the dial tone database will be created.

Read the full post here.

-Thanks, Jinesh.