OK so yes, externally it's clearly labelled as a Police Public Call Box. But it's not a telephone box. It's certainly not a phone booth. It's not even a Police Box. The Police Box thing is just, if you like, an outer shell. It's how the ship is rendered to the outside world. Inside it's totally different. And much bigger, just in case you hadn't noticed that yet. It's actually meant to be able to blend into its surroundings, kinda like a chameleon. In fact one of the TARDIS's core components is a thing called the Chameleon Circuit. It's a kind of cloaking device which, when functioning correctly, allows the TARDIS to land anywhere and essentially go unnoticed by materialising in the form of an object that's congruous with the surroundings. But the Doctor's Chameleon Circuit doesn't work and hasn't done for the vast majority of Doctor Who's fifty years. Despite how clever The Doctor is, despite having come into contact with loads of other Time Lords along the way and despite having returned to his home planet of Gallifrey oodles of times over the years, for some reason he's never worked out how to fix his faulty one, nor managed to track down a new one. It would kinda kill the magic if it didn't look like a phone booth though.