Unable to disable user

Hello, I am having an issue where I am unable to Disable a user. We just had someone leave the company recently and I need to re-purpose their number and assigned to a new hire coming onboard. Typically, what I do is Disable the user then Delete the user and put the number back into the Number Inventory. When I try to Disable the use in question, I receive an error message that says "Some users cannot be Disabled. You need to only select users that are in Enabled state".

Any help would be greatly appreciated. I've submitted a web case but support is non-existent.