Computed Column Operand type clash on Date Column When Checking For Null
According to the ISNULL documentation the returned data type is the same type as the check expression, which is data type date in this case. The ISNULL expression fails because the integer literal 1 cannot be implicitly converted to date.

To return a bit indicating whether or not the date column is NULL, you could instead use a CASE expression:

CREATE TABLE [history].[PhoneBook](
    [PhoneBookId] [int] IDENTITY(1,1) NOT NULL,
    [PersonId] [int] NOT NULL,
    [PhoneId] [int] NOT NULL,
    [DateActive][Date] NOT NULL,
    [DateInActive][Date] NULL,
    [IsValid] AS CASE WHEN DateInActive IS NULL THEN CAST(0 AS BIT) ELSE CAST(1 AS bit) END
 CONSTRAINT [PK_PhoneBook_1] PRIMARY KEY CLUSTERED 
(
    [PersonId] ASC,
    [PhoneId] ASC

)WITH (PAD_INDEX = OFF, STATISTICS_NORECOMPUTE = OFF, IGNORE_DUP_KEY = OFF, 
       ALLOW_ROW_LOCKS = ON, ALLOW_PAGE_LOCKS = ON) ON [PRIMARY]
) ON [PRIMARY]