can anybody please explain me the difference between call and data filters? manual plainly says that call filter are used when WAN is down. the thing i cannot understand is why we may need to setup some filters when there is no internet connection at all? what would we filter?

The call filter is not so useful these days, it's used for deciding what criteria would make the WAN connection dial-up when it wasn't set to "Always On"

you know it for sure?

if i use always-on static ip wan address, i will never need call filter?

You'll probably never need the call filter anyway, unless you've got an idle timeout set on your internet connection, there's no point in configuring it.