My guess is that your provider is sending empty or nonsense packets to keep the NAT asscociation open "keep-alive". You can confirm that with SIP Debug. If you don't actually need those packets, there may be a setting on the provider site to turn them off, a request to their support may get them turned off, or possibly they won't be sent if you set up NAT mapping.
However, it's IMO better to live with the bogus log entries, rather than make a change that might degrade the reliability of your system.
