sip: prevent checkNATAddress refresh registeration immediately

- For OVH sip trunk, if too many registeration requests are sent
  at the same time, it will send back a 500 status code to prevent
  us sending registeration request in certain time period.

- That means at the starting of the programme, the account is not
  registered, which is the cause of the issue #643

Change-Id: I89dd7331704c6abdf1021c3e3cecd2b55516c4d1