The BTWifi-Fon is usually transmitted from a BT customers Homehub. When there are problems logging onto a signal that is being broadcast it is probably because the Homehub is needing to be re-set.
BT can not do this so unless you know where the signal is being broadcast from and feel you can approach the owner of the Homehub and ask him/her to reset it, there is nothing that can be done.
Is the FON you're connecting to actually your own Home Hub that hasn't got a connection to the internet?
Just to add to this I'm also having probs with FON in my flat im in for too short of a time to sort having a phone line installed etc anyways been using fon here since december and was connected fine up until 2 weeks ago when the signal got stronger but then started going limited every 10 mins or so theres 2 new BT hubs close with full 5 bars but when I have the limited issues its says im connected to BT WIFI with FON 10 but it works perfectly when on BT WIFI with FON 5 7 or 11 sounds crazy as im in the same location that it connects to different numbers what ever they mean if anyone has a solution so i connect to anything but 10 id appreciate it