Calculate time not logged in

I think you could use a PivotTable type of report to summarize by the amount of "logged out" time for each extension.

If you sort the data in ascending order by date, extension & time, you can calculate the amount of logged out time preceding each session using a formula like:
=C2-IFERROR(AGGREGATE(15,6,D$1:D1/((A$1:A1=A2)*(B$1:B1=B2)),1),TIME(9,0,0))
The preceding formula requires Excel 2010 or later. Please advise if you are using an earlier version of Excel, as alternative formulas are possible.
DowntimerQ28588750.xlsm

If you use Excel 2007, you can use an array-entered formula instead of the previously suggested AGGREGATE formula:
=C2-MAX(IFERROR(MIN(IF((A$1:A1=A2)*(B$1:B1=B2)=1,D$1:D1,"")),0),TIME(9,0,0))

Here is code that illustrates how to use both formulas to produce a PivotTable report:

Sub Downtimer()
Dim rg As Range, rgdata As Range
Application.ScreenUpdating = False
With ActiveWorkbook.Worksheets(1)
    .Range("E1") = "Downtime"
    Set rg = .Range("A1").CurrentRegion
    Set rgdata = rg.Cells(2, 1).Resize(rg.Rows.Count - 1, 5)
    
        'Use either the next statement (Excel 2010 or later) or the two that follow it (Excel 2007)
    'rgdata.Columns(5).FormulaR1C1 = _
        "=RC[-2]-IFERROR(AGGREGATE(15,6,R1C[-1]:R[-1]C[-1]/((R1C[-4]:R[-1]C[-4]=RC[-4])*(R1C[-3]:R[-1]C[-3]=RC[-3])),1),TIME(9,0,0))"
    rgdata.Cells(1, 5).FormulaArray = _
        "=RC[-2]-MAX(IFERROR(MIN(IF((R1C[-4]:R[-1]C[-4]=RC[-4])*(R1C[-3]:R[-1]C[-3]=RC[-3])=1,R1C[-1]:R[-1]C[-1],"""")),0),TIME(9,0,0))"
    rgdata.Cells(1, 5).AutoFill Destination:=rgdata.Columns(5)
    
    rgdata.Columns(5).NumberFormat = "h:mm:ss;@"
    .Sort.SortFields.Clear
    .Sort.SortFields.Add Key:=rgdata.Columns(1), SortOn:=xlSortOnValues, Order:=xlAscending, DataOption:=xlSortNormal
    .Sort.SortFields.Add Key:=rgdata.Columns(2), SortOn:=xlSortOnValues, Order:=xlAscending, DataOption:=xlSortNormal
    .Sort.SortFields.Add Key:=rgdata.Columns(3), SortOn:=xlSortOnValues, Order:=xlAscending, DataOption:=xlSortNormal
    With .Sort
        .SetRange rg
        .Header = xlYes
        .MatchCase = False
        .Orientation = xlTopToBottom
        .SortMethod = xlPinYin
        .Apply
    End With
End With
PTmaker rg
End Sub

Sub PTmaker(rg As Range)
Dim shName As String
Dim wb As Workbook
With rg.Worksheet
    Set wb = .Parent
    shName = "'" & .Name & "'!"
    wb.PivotCaches.Create(SourceType:=xlDatabase, SourceData:=shName & rg.Address(ReferenceStyle:=xlR1C1)).CreatePivotTable _
        TableDestination:=shName & "R1C8", TableName:="PivotTable1"
    With .PivotTables("PivotTable1")
        With .PivotFields("Extn")
            .Orientation = xlColumnField
            .Position = 1
        End With
        With .PivotFields("Date")
            .Orientation = xlRowField
            .Position = 1
        End With
        .AddDataField .PivotFields("Downtime"), "Sum of Downtime", xlSum
        With .PivotFields("Downtime")
            .Orientation = xlRowField
            .Position = 2
        End With
        .DataBodyRange.NumberFormat = "h:mm:ss;@"
    End With
End With
End Sub

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A sample workbook using the code is attached. As written, the code applies to the active workbook (i.e. the macro may be located in your Personal.xlsm or other workbook).
DowntimerQ28588750.xlsm