Sorry Guys, I'm very new to this. Taking a dive 'under the hood', to try to learn what is going on, and to see what I can achieve.
I have 1 sip device ( voip telephone) on the network, constantly firing out REGISTER requests (I have turned off my asterisk server!). That's good.
I have a terminal window open, running tcpdump. That's good too - I can see what is going on.