At your Update:
UPDATE [dbo].[PersonPhone]
SET [StartDate] = DATEADD(day, 1, [HumanResources].[EmployeeDepartmentHistory].[StartDate])
FROM [HumanResources].[EmployeeDepartmentHistory]
GO
SELECT row_number() over(order by (select 1)) AS [RN] FROM [HumanResources].[EmployeeDepartmentHistory]