Dim Lastrow As Long, i As Long
Lastrow = Cells(Rows.Count, 11).End(xlUp).Row
For K = 2 To Lastrow
If Not IsEmpty(Cells(K, 11).Value) Then Cells(K, 11).Value = "Tel.: " & Cells(K, 11).Value
Next K
Lastrow = Cells(Rows.Count, 12).End(xlUp).Row
For L = 2 To Lastrow
If Not IsEmpty(Cells(L, 12).Value) Then Cells(L, 12).Value = "Ext.: " & Cells(L, 12).Value
Next L
Lastrow = Cells(Rows.Count, 14).End(xlUp).Row
For N = 2 To Lastrow
If Not IsEmpty(Cells(N, 14).Value) Then Cells(N, 14).Value = "Cel.: " & Cells(N, 14).Value
Next N
Lastrow = Cells(Rows.Count, 13).End(xlUp).Row
For i = 2 To Lastrow
If Not IsEmpty(Cells(i, 13).Value) Then Cells(i, 13).Value = "Fax: " & Cells(i, 13).Value
Next i
Case Else
Exit Sub
End Select
End Sub