Exchange 2010. Cannot disable UM for user

I tried to do it using Control panel and Powershell, but it does not work:

[PS] C:\Windows\system32>Disable-UMMailbox -Identity User@domain.com

Confirm
Are you sure you want to perform this action?
The UM mailbox "User@domain.com" is being disabled.
[Y] Yes  [A] Yes to All  [N] No  [L] No to All  [?] Help (default is "Y"): a
Mailbox 'User@domain.com' couldn't be disabled for Unified Messaging : Cannot save changes made to an
 item to store.
    + CategoryInfo          : NotSpecified: (:) [Disable-UMMailbox], RecipientTaskException
    + FullyQualifiedErrorId : 47895907,Microsoft.Exchange.Management.Tasks.UM.DisableUMMailbox

[PS] C:\Windows\system32>

Can you help?

Thank you.

Thank you. Eric.