No lock, but a lot of the ones I remember had a bifold door with the hinge on the inside, so you could lean against it to effectively keep it shut. It was possible to wedge something in there to keep the door shut, and I think that was the subject of a scene in a slapstick movie, where the poor schmuck trying to use the phone left his briefcase perched on a shelf, and as he left it fell and wedged the door shut. I can't remember the movie, though.

There were no locks of the booths, and phones were in fact occasionally stolen. But it was generally not worth the hassle, and so there were no locks. Plus they were meant to be accessible 24/7.

Finally, many times the phones weren't in the booth and we're just mounted to a wall. The booth was to give you some theoretical privacy and reduce background noise, that's about it.

Boy do I have a movie for you

In my region it was a common prank among youths to lock people there by driving your car very closely up to the door :-)

Watch the documentary with Kiefer Sutherland called Phone Booth. That’s how you get stuck in a phone booth.

I highly doubt it. I don't remember any of incidents.

Source: Old millennial born in the 1980s

I dont recall any (British) telephone booths having locks

They didn't have locks but you could probably break the hinges on the door in a way that it doesn't open, maybe? I've only ever seen like a folding door (double hinged) design. Maybe some pennies? It'd work for a normal door, not sure about the accordion style tho.

There's a movie called "Phonebooth" that has a dude trapped in a phonebooth because a sniper is watching him and blackmailing him because he's cheating on his wife/fiance. That's the most notable a thing I know involving a phone booth other than Dr. Who.

Used to be a college challenge to stuff as many people in one as possible; I bet you could find a horror story or two stemming from that, perhaps.