[intefix]

Ψάχνοντας βρήκα μια καλή λύση...
Αν το σύνθετο πλαίσιο το το πούμε phone
έχουμε...

Private Sub phone_Change()
Dim strRowSource As String

  strRowSource = "SELECT person_id, phone, l_name, f_name, address  FROM ref_person"
  If Len(Me!phone.Text) = 5 Then
     strRowSource = strRowSource & " WHERE phone Like '"
     strRowSource = strRowSource & Me!phone.Text
     strRowSource = strRowSource & "*'"
     Me!phone.RowSource = strRowSource
     Me!phone.Dropdown
  End If
End Sub