Sum of values from specific rows

You can pivot on the hours and perform row level operations you would like:

Advanced Editor Code:

let
    Source = #"Original Source",
    Types = 
        Table.TransformColumnTypes(Source,{{"InboundCalls", Int64.Type}}),
    Pivot = 
        Table.Pivot(Types, List.Distinct(Types[Hour]), "Hour", "InboundCalls", List.Sum),
    ApplyLogic = 
        Table.FromRecords( 
            List.Transform( 
                Table.ToRecords( Pivot ), 
                (row) => Record.TransformFields( row, {{"7", each 0},{"8", each row[7] + _}} ) 
            ), 
            Value.Type(Pivot) 
        ),
    Unpivot = 
        Table.UnpivotOtherColumns(ApplyLogic, {"Date"}, "Hour", "InboundCalls")
in
    Unpivot

For explanation/convo on the technique used in ApplyLogic step, see https://stackoverflow.com/questions/31548135/power-query-transform-a-column-based-on-another-column

(edit: added screenshot of steps in query editor)