They've stood on the streets of London for nearly a century (This one is from the 1920s). But realistically, they probably don't have a long future ahead of them now that most people have mobile phones that do the Internet and video conferencing, let alone simple phone calls.
They could be converted into Internet kiosks, but it remains to be seen if smartphones will put an end to that idea. BT's first attempt at public Internet kiosks in the early '00s didn't go very far. And besides, these large iron boxes probably aren't what the companies wanting to install Internet kiosks want to use.
They can't just be disconnected and left where they stand as street furniture because no one's going to accept responsibility for maintaining and insuring them etc. It's more likely that BT will take the clinical decision to dispose of 'redundant assets', just as casually as the analogue exchange equipment it throw out in the 80s.
So like the equally iconic red London Routmaster buses, one day they'll probably all but disappear, certainly outside of central London.
Perhaps all the phone boxes around Westminster should be temporarily painted like this for the Royal wedding of Prince William and Kate Middleton? Each box could be decorated in a different flag of a Commonwealth nation.