Deleted Resource account with phone number still assigned.

If you delete a resource account without first removing the phone number you can run Get-CsPhoneNumberAssignment -TelephoneNumber <+phonenumber> and note the AssignedPstnTargetId which is the Identity of the deleted Resource Account.

Then run the following to remove the number.
Remove-CsPhoneNumberAssignment -Identity <AssignedPstnTargetId> -PhoneNumber <+phonenumber to unassign> -PhoneNumberType <DirectRouting|CallingPlan|OperatorConnect>