Hmmm in Firewall - "Such page allows you to enable / disable Call Filter and Data Filter, determine general rule for filtering the incoming and outgoing data" - presume "Call" here means data packets coming and going?

The "Call Filter" is actually used to determine what traffic is allowed to bring up the WAN interfaces, if the WAN interface is PPP and isn't set to "Always On" (which all internet connections typically are now). If you set up a block rule in the Call Filter, that traffic will be ignored and will not bring up the internet connection if it tries to access the internet.
Data Filter is the actual firewall part, rules there determine which traffic is allowed to pass through the router.

Ah- thanks - and I can also see a set of filters - where for example I could block one or a range of IP's

Interesting you have xNetBios -> DNS set as a default - why this one?