Set your X_UserAgentPort to a random number between 20000 and 65535, for example, 58192.
This will make it more difficult for the scanners to find you, but not impossible. The best solution is to place your device behind a firewall. A restricted cone NAT firewall will only allow traffic from your VoIP service provider. All other traffic will be dropped, before it reaches your ATA. My favourite firewall is a Tomato router.
Thanks Mango, so I can just try this setting first:
Set your X_UserAgentPort to a random number between 20000 and 65535, for example, 58192