Prank phone calls. Its still possible with enough dedication and effort, but between smart call ID, the phone boxes disappearing and 99.99% of incoming calls being scams its just not worth the effort.

Time was if you knew a phone number and 30p in your pocket you could pop into a phone box and get an expensive laugh.

I also kinda miss how they smell. It was awful mix of piss, cigarettes and burnt plastic, but it was a pretty unique sensation.