Exchange 2010 Unified Messaging – Unrecognised Voicemail Extension


I had an issue where a particular user’s voicemail on Exchange 2010 wasn’t working. When they called the voicemail number, they were asked “To access your mailbox, please enter the extension” rather than being greeted with their name and messages. The user was on Lync 2010, and Unified Messaging was enabled on their account with the matching extension number. Also, when dialing voicemail and trying to enter the extension number just met the message ‘XXX isn’t a valid mailbox’.

Disabling and re-enabling Unified Messaging made no difference. I was rather confident this was an Exchange/Unified Messaging issue, so I had the idea of checking ADSI edit to see if UM was properly enabled.

Checking the attribute “msExchUMEnabledFlags” found the issue. When a user has UM enabled, the value of this field should be 831, or 830 when disabled. I’ve come across this issue before, where it’s the wrong value and needs to be changed, but this was the first time I’d come across it as being 832.

Never a good sign when you google something, and you get zero results:


Changing the Attribute value back to 831 instantly fixed the issue, didn’t even require disabling and re-enabling Unified Messaging. If all else fails, change the value back to 830, refresh your Exchange Management Console and the user should show as being disabled for Unified Messaging, then re-enable as you normally would.

Hope this helps anyone else who comes across a similar issue.

5 thoughts on “Exchange 2010 Unified Messaging – Unrecognised Voicemail Extension

  1. Glad you created this post, I was having this exact issue, and your write up is the only reference I could find.

  2. Thanks for this information; it may be useful in the future as I didn’t appreciate the Attributes on the account. In my instance we have a generic account used for SIP / Email / UM – tried allsorts to try and resolve, including the above. In the end it was simply that the AD account had locked out, not obvious as the user isn’t logging on as that account for network profile. As soon as we unlocked the account, hey presto, it started working again!!

  3. Came across this post/solution when looking for a similar issue with the extension being invalid – we have an Exchange 2016 on-premise and 365 hybrid environment where the on-premise users use UM for voicemail in Exchange. As it is right now, the 365 mailboxes don’t have voicemail functionality so I had to migrate a handful of users back to the on-premise server. After doing this, I assigned the phone extensions as I usually do but when trying to call into the voicemail box, we’d just get “That is not a valid extension.”. We could assign that same extension to a mailbox that was only in on-premise and not in 365 and it would work correctly. Sure enough, the attribute “msExchUMEnabledFlags” was set to “1” and the “msExchUMEnabledFlags2” was set to “-1”. After confirming/setting the correct value “831” on the “msExchUMEnabledFlags” attribute and clearing the value on “msExchUMEnabledFlags2” the voicemail box worked as expected. Thank you for posting this almost 10 years ago!

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