"Extension number can‎'t be generated for the user" error when you try to move a mailbox from the on-premises environment to Office 365

PROBLEM
When you try to on-board or move a mailbox from an on-premises Exchange Server environment to Exchange Online in Office 365 in a hybrid deployment, the operation fails during the validation stage. You receive the following error message:
MigrationPermanentException:
Mailbox '<UserName>' in the source forest is currently enabled for Unified Messaging but it can‎'t be enabled for Unified Messaging in the target forest for the following reason: An extension number can‎'t be generated for the user. To configure an extension number, use the -Extensions parameter with one or more valid extension numbers. Please fix the problem or disable the mailbox for Unified Messaging before you try the operation again.
CAUSE
This issue occurs if the mailbox is missing a primary Exchange Unified Messaging (EUM) address.

The New-MoveRequest cmdlet that's used in the mailbox move procedure performs some validation actions. One of these actions is to check whether the source mailbox is enabled for Unified Messaging. To confirm that this is the issue that you're experiencing, check whether the source on-premises mailbox has a proxy address that contains an "EUM" (uppercase letters) prefix. To do this, follow these steps:
  1. Open the Exchange Management Shell, and then run the following commands:
    1. Get-mailbox user@contoso.com |select -expand EMailAddresses
    2. Get-UMMailbox user@contoso.com |fl
  2. Examine the output. If the mailbox is Session Initiation Protocol (SIP)–enabled, the mailbox should have a primary "EUM" (uppercase letters) proxy address and a secondary "eum" (lowercase letters) proxy address. If the primary EUM address is missing, you've confirmed that this is the issue that you're experiencing.
SOLUTION
Manually add the primary EUM address. For example, if the mailbox is SIP-enabled, it should have two entries that look like this:

EUM:firstname.surname@<domain>.com;phone-context=DialPlanName.global.loc
eum:<phone extension>;phone-context=DialPlanName.global.loc
For more information about how to add an EUM address, see Manage user mailboxes.
MORE INFORMATION
Still need help? Go to the Office 365 Community website or the Exchange TechNet Forums.
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Article ID: 3142072 - Last Review: 02/24/2016 20:04:00 - Revision: 2.0

Microsoft Exchange Online, Exchange Server 2016 Enterprise Edition, Exchange Server 2016 Standard Edition, Microsoft Exchange Server 2013 Enterprise, Microsoft Exchange Server 2013 Standard

  • o365e o365m o365022013 o365 o365a hybrid KB3142072
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