Sub Whatever
  Dim Ns As Outlook.NameSpace
  Dim Contacts As Items
  Dim Contact As ContactItem

  Set Ns = Application.GetNamespace("MAPI")

  'use the default folder
  Set Contacts = Ns.GetDefaultFolder(olFolderContacts).Items.Restrict("[MessageClass]='IPM.Contact'")

  'do whatever
  On Error GoTo WTF

  For Each Item In Contacts
    If Item.Class = olContact Then
      Set Contact = Item
      With Contact
        ' Call SetCategories(Contact)
        '.MobileTelephoneNumber = FormatTel(.MobileTelephoneNumber)
        '.AssistantTelephoneNumber = FormatTel(.AssistantTelephoneNumber)
        '.Business2TelephoneNumber = FormatTel(.Business2TelephoneNumber)
        '.BusinessFaxNumber = FormatTel(.BusinessFaxNumber)
        '.BusinessTelephoneNumber = FormatTel(.BusinessTelephoneNumber)
        '.CallbackTelephoneNumber = Format(.CallbackTelephoneNumber)
        '.CarTelephoneNumber = FormatTel(.CarTelephoneNumber)
        '.CompanyMainTelephoneNumber = FormatTel(.CompanyMainTelephoneNumber)
        '.Home2TelephoneNumber = FormatTel(.Home2TelephoneNumber)
        '.HomeFaxNumber = FormatTel(.HomeFaxNumber)
        '.HomeTelephoneNumber = FormatTel(.HomeTelephoneNumber)
        '.OtherFaxNumber = FormatTel(.OtherFaxNumber)
        '.OtherTelephoneNumber = FormatTel(.OtherTelephoneNumber)
        '.PagerNumber = FormatTel(.PagerNumber)
        '.PrimaryTelephoneNumber = FormatTel(.PrimaryTelephoneNumber)
        '.RadioTelephoneNumber = FormatTel(.RadioTelephoneNumber)
        '.TTYTDDTelephoneNumber = FormatTel(.TTYTDDTelephoneNumber)

        ' prioritise
        PrioritySwitch .MobileTelephoneNumber, .BusinessTelephoneNumber, True
        PrioritySwitch .MobileTelephoneNumber, .Business2TelephoneNumber, True
        PrioritySwitch .MobileTelephoneNumber, .BusinessFaxNumber, True
        PrioritySwitch .MobileTelephoneNumber, .CompanyMainTelephoneNumber, True
        PrioritySwitch .MobileTelephoneNumber, .PrimaryTelephoneNumber, True
        PrioritySwitch .MobileTelephoneNumber, .AssistantTelephoneNumber, True
        PrioritySwitch .MobileTelephoneNumber, .CallbackTelephoneNumber, True
        PrioritySwitch .MobileTelephoneNumber, .CarTelephoneNumber, True
        PrioritySwitch .MobileTelephoneNumber, .CompanyMainTelephoneNumber, True
        PrioritySwitch .MobileTelephoneNumber, .Home2TelephoneNumber, True
        PrioritySwitch .MobileTelephoneNumber, .HomeTelephoneNumber, True
        PrioritySwitch .MobileTelephoneNumber, .OtherTelephoneNumber, True
        PrioritySwitch .MobileTelephoneNumber, .PagerNumber, True
        PrioritySwitch .MobileTelephoneNumber, .HomeFaxNumber, True
        PrioritySwitch .MobileTelephoneNumber, .OtherFaxNumber, True

        If Not .Saved Then .Save
      End With
    End If
  Next
  Exit Sub
  WTF:
    Debug.Print Contact
End Sub

Sub PrioritySwitch(ByRef Tel1 As String, ByRef Tel2 As String, mobileOnly As Boolean)
  Dim t1 As String
  Dim switchTel As Boolean

  switchTel = False

  'Logic code goes here
  switchTel = True

  If switchTel Then
    t1 = Tel1
    Tel1 = Tel2
    Tel2 = t1
  End If
End Sub

With Contact
    '...
    PrioritySwitch .MobileTelephoneNumber, .BusinessTelephoneNumber, True

Dim stackVar1 As String
stackVar1 = Contact.MobileTelephoneNumber
Dim stackVar2 As String
stackVar2 = Contact.BusinessTelephoneNumber
PrioritySwitch stackVar1, stackVar2, True
'...and then stackVar1 and stackVar2 disappear...

Public Property Let MobileTelephoneNumber(number As String)
    'Frobs whatever Outlook does to store the number.
End Property

Public Property Get MobileTelephoneNumber() As String
    'Frobs whatever Outlook does to retrieve the number.
End Property

Public Sub PrioritySwitch(target As ContactItem, propertyOne As String, _
                          propertyTwo As String, mobileOnly As Boolean)
    Tel1 = CallByName(target, propertyOne, VbGet)
    Tel2 = CallByName(target, propertyTwo, VbGet)

    '...code that sets switchTel

    If switchTel Then
      CallByName target, propertyOne, VbLet, Tel2
      CallByName target, propertyTwo, VbLet, Tel1
    End If
End Sub

' prioritise
    PrioritySwitch Contact, "MobileTelephoneNumber", "BusinessTelephoneNumber", True
    PrioritySwitch Contact, "MobileTelephoneNumber", "Business2TelephoneNumber", True
    PrioritySwitch Contact, "MobileTelephoneNumber", "BusinessFaxNumber", True
    PrioritySwitch Contact, "MobileTelephoneNumber", "CompanyMainTelephoneNumber", True
    ' etc...