Windows 7 Wireless Network Won't Allow Internet Acces

If you have three wireless computers that suddenly cannot access the internet through your Telkom Billion 400G router then I would first suspect the wireless radio in the router. But if as you seem to suggest that all three computers can still communicate with each other than the problem might be with your DSL provider or the link port on the router that you connect the phone line to.
You might first start by recycleing the router, shut all the computers down first then turn it off and remove the power. Wait a while (couple minutes) then restore the power and turn the power switch back on, wait again a brief period before bring the computers back up one at a time testing the connection as you go. If that has no effect then you might try resetting the router with the reset switch on the back, normally recessed inside a small hole. Read the router documentation for exactly how to do this and make sure you have any connection specific information required written down some place before you do it (PPPoE connections will often require a username and password to re-establish the connection after a factory reset).
The router has four LAN ports, have you tried a direct cat5 wired connection to see if that works? Or perhaps a call to your provider may help, in case they are having network issues in your area.