I noticed that the behaviour of the NAT_Keep_Alive_Msg sent by the device changed with firmware 5.2.10.
Prior to 5.2.10 the keep alive messages did not contain a 'Contact'-Header. Changing the message type from $NOTICE to $REGISTER worked fine with Asterisk.
Now the messages do contain a 'Contact'-Header, but with ";expire=0" attached to the end. When the message type is set to $REGISTER this causes Asterisk to UNregister the client, what surely is not the sense of a keep alive message! Guess what happend after updating some of my customers... not really worth a smile! :-(
Assuming that this is a bug and will hopefully be fixed with the next release I strongly suggest to stay with the default value of $NOTICE when there is no real need to change it. To the development team I suggest to interpret the $REGISTER type to send out just an ordinary REGISTER message as is normally sent before expiry, or simply keep the old behaviour as it worked out with Asterisk.
Feedback to this comment is strongly encouraged :-)