In Skype for Business admin center, you have the option to use one of your Service Numbers as default Conference Bridge number instead of the default shared number.
However, if you changed your mind and wish to unassign this number and use it for a different purpose, you might notice that the Unassign button is greyed-out and while hoovering over it, you are redirected to contact Support.
There is a workaround to do this by yourself, and all you need is Windows PowerShell and the Skype for Business Online Windows PowerShell Module available here.
1.Connect to Skype for Business Online by using Windows PowerShell. (https://technet.microsoft.com/en-us/library/dn362795(v=ocs.15).aspx)
2.Obtain the name of the Bridge by running the following cmdlet:
3. Unregister-CsOnlineDialInConferencingServiceNumber -BridgeName “Conference Bridge” -RemoveDefaultServiceNumber 1234 // where 1234 should be replaced by the number you wish to unregister.
4. Go back to the Skype for Business admin center and check if the number has been unassigned: