To show max value based on condition within time period

let
  Source = Table.FromRows(
    Json.Document(
      Binary.Decompress(
        Binary.FromText(
          "ddBBDsQgCIXhq0xc1wQeqJWrNL3/NUY70tLFJO6+/BA8jsTMaUvEmTlDPgwrPZ2bA5c/gLsoVmoEcVCjEqE5NKPXqP0ZtQDABM2MC9igEbrDKCgASR6nQOcO1ffyBezFuhx5vAndiCPUBSCDRGheVJNfISLzr+gB7AFwAy44vw==", 
          BinaryEncoding.Base64
        ), 
        Compression.Deflate
      )
    ), 
    let
      _t = ((type nullable text) meta [Serialized.Text = true])
    in
      type table [Phone = _t, Date = _t]
  ), 
  #"Changed Type" = Table.TransformColumnTypes(
    Source, 
    {{"Phone", Int64.Type}, {"Date", type datetime}}, 
    "en-GB"
  ), 
  #"Grouped Rows" = Table.Group(
    #"Changed Type", 
    {"Phone"}, 
    {{"Rows", each _, type table [Phone = nullable number, Date = nullable datetime]}}
  ), 
  Process = (tbl) =>
    let
      #"Sorted Rows" = Table.Sort(tbl, {{"Date", Order.Ascending}}), 
      #"Added Index" = Table.AddIndexColumn(#"Sorted Rows", "Index", 0, 1, Int64.Type), 
      #"Added Custom" = Table.AddColumn(
        #"Added Index", 
        "Group", 
        each List.Accumulate(
          {0 .. [Index]}, 
          1, 
          (state, current) =>
            if current = 0 then
              state
            else if #"Added Index"{current}[Date]
              - #"Added Index"{current - 1}[Date] > #duration(7, 0, 0, 0)
            then
              state + 1
            else
              state
        )
      ), 
      #"Removed Other Columns" = Table.SelectColumns(#"Added Custom", {"Phone", "Date", "Group"}), 
      #"Grouped Rows1" = Table.Group(
        #"Removed Other Columns", 
        {"Group"}, 
        {
          {
            "Rows", 
            each Table.AddIndexColumn(_, "Index", 1, 1, Int64.Type), 
            type table [
              Phone = nullable number, 
              Date = nullable datetime, 
              Group = number, 
              Index = Int64.Type
            ]
          }, 
          {"Max", each Table.RowCount(_), Int64.Type}
        }
      ), 
      #"Expanded Rows" = Table.ExpandTableColumn(
        #"Grouped Rows1", 
        "Rows", 
        {"Phone", "Date", "Index"}
      )
    in
      #"Expanded Rows", 
  #"Added Custom" = Table.AddColumn(#"Grouped Rows", "Processed", each Process([Rows])), 
  #"Expanded Processed" = Table.ExpandTableColumn(
    #"Added Custom", 
    "Processed", 
    {"Group", "Date", "Index", "Max"}, 
    {"Group", "Date", "Index", "Max"}
  ), 
  #"Removed Other Columns" = Table.SelectColumns(
    #"Expanded Processed", 
    {"Phone", "Group", "Date", "Index", "Max"}
  )
in
  #"Removed Other Columns"

How to use this code: Create a new Blank Query. Click on "Advanced Editor". Replace the code in the window with the code provided here. Click "Done". Once you examined the code, replace the Source step with your own source.