Extension number can't be generated for the user when moving a mailbox from on-premises to Office 365
Original KB number: 3142072
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:
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
-Extensionsparameter with one or more valid extension numbers. Please fix the problem or disable the mailbox for Unified Messaging before you try the operation again.
This issue occurs if the mailbox is missing a primary Exchange Unified Messaging (EUM) address.
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:
Open the Exchange Management Shell, and then run the following commands:
Get-mailbox email@example.com |select -expand EMailAddresses
Get-UMMailbox firstname.lastname@example.org |fl
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.
Manually add the primary EUM address. For example, if the mailbox is SIP-enabled, it should have two entries that look like this:
For more information about how to add an EUM address, see Manage user mailboxes.