using statement "case" using vba program

Hi Novri, and welcome to the forum.

The Select Case statement shown below should be what you're looking for:

Private Sub Worksheet_Change(ByVal Target As Range)
    If Not Intersect(Target, Range("A1")) Is Nothing Then
        Select Case Range("A1").Value
            Case 812 To 815
                Range("B1").Value = "Australia"
            Case 511, 611, 711, 845, 852, 911
                Range("B1").Value = "Bangkok"
            Case Else
                Range("B1").Value = "Roamer"
        End Select
    End If
End Sub