use linq for your solution

In str string variable your result for unique phone nos and their last call timings

also refer to this http://msdn.microsoft.com/en-us/library/bb386922.aspx#Y272

class

Imports Microsoft.VisualBasic

Public Class PhoneCalls
    Dim _phoneno As String
    Dim _calltime As Date
    Public Property phoneno() As String
        Get
            Return _phoneno
        End Get
        Set(value As String)
            _phoneno = value

        End Set
    End Property

    Public Property calltime() As Date
        Get
            Return _calltime
        End Get
        Set(value As Date)
            _calltime = value

        End Set
    End Property
    Sub New()

    End Sub
    Sub New(_phoneno As String, _calltime As Date)
        phoneno = _phoneno
        calltime = _calltime
    End Sub
End Class

Imports PhoneCalls
Imports System.Linq

Partial Class Default1
    Inherits System.Web.UI.Page


    Protected Sub Page_Load(sender As Object, e As System.EventArgs) Handles Me.Load

        Dim pc As PhoneCalls() = {New PhoneCalls("123", Date.Now), New PhoneCalls("456", Date.Now.AddDays(-1)),
                                 New PhoneCalls("123", Date.Now.AddDays(1))}

        Dim pc1 As List(Of PhoneCalls)
        pc1 = pc.ToList()

        Dim query = From p In pc1
                    Group p By p.phoneno Into g = Group
                    Select New With {g, .calltime = g.Max(Function(p) p.calltime)}

        Dim str As String = ""
        For Each q In Query
            str += " " + q.g.First(Function(p) p.phoneno).phoneno.ToString() + " - " + q.calltime.ToString()
        Next

    End Sub


End Class