Openzone and fon hotspots will only show when you are in range of one, if you are out of range then you can't log on!
Simply logging onto the wireless network doesn't use your credit, you log onto the network then try to browse to any web page and it will take you to the logon page automatically where you have to enter your logon details, once you've done that you can browse the web normally. It's suggested you use another tab for browsing and leave the BT fon page open because then it's easier to log out when you've finished (though I'm sure it's meant to auto log you out after about 20min of inactivity if you forget)