Just like a photo booth, you'd plonk your coin in the slot, but instead of posing for some snaps, you'd start speaking.
The idea was that those who were travelling could send a voice message from the location they were visiting - a bit like a sonic postcard!
2/6 (13p) for a 3 minute recording at the standard speed of 45rpm.
Mind you, it was also put to use by the locals from time to time.
Yvonne J (Susan Childe from The Flamingos) was telling me that when she was a young teenager she once popped into the booth and recorded a full song - a bit of a precursor to her later years in proper recording studios.
The booth in the picture isn't the Dundee one, but I'm sure every main town in Britain had one back then.
I guess they must have been phased-out some time in the mid/late 70's.