The disadvantage of the chamelion circuit is that it makes the TARDIS "look" like "something" and there's always the risk that "someone" could need, or try to enter that "something." You essentially end up with the reverse of the gag from the start of this year's Christmas special. Some poor copper tries to call his wife to say he's working late and ends up getting an eye-full of TARDISy goodness.
Except that most of the other cited forms for TARDISes haven't been things you'd expect to enter. The Master's grandfather-clock and Roman-column versions have been mentioned upthread, as have the forms the Doctor's TARDIS took in "Attack of the Cybermen," including a pipe organ and a stove. In "An Unearthly Child," Susan said that the TARDIS had previously been an Ionic column and a sedan chair (okay, a sedan chair is something one could climb into). The Meddling Monk's TARDIS appeared as a sarcophagus, a large boulder, a carved stone block, and various other forms.
Anyway, the telephone in a police box only connected to the station, so it couldn't be used for personal calls. It was a way for the public to contact the police station or for police to check in. The light on top was to signal officers in the area to call the station, or for the public to signal them in an emergency. And you didn't need to enter the police box interior to use the phone; you just open the hatch with the sign on it (despite what Idris said in "The Doctor's Wife," the PULL TO OPEN sign refers to the phone hatch that it's printed on, not to the big doors). The interior contained various supplies like an incident book, a fire extinguisher, and a first-aid kit. I read once that they could be used to lock up suspects until a wagon could arrive to take them in, but I can't find any confirmation of that at Wikipedia or the other online sources I checked.
^or perhaps the TARDIS has just decided shes likes that look and doesn't want to change. Given that she takes the Doctor where he needs to be. Usually where there is someone/thing doing something wrong. The Police box does sort of fit in with what the Doctor does. He fights injustice etc.. across time and space.
Yup. "Free for Use of Public. Advice and Assistance Obtainable Immediately. Officers and Cars Respond to Urgent Calls." That's a pretty good mission statement for the Doctor.
Plus the Hartnell, Cushing, and Smith versions bear the St John Ambulance logo, which is kind of like the Red Cross, the symbol of a group of charitable organizations dedicated to teaching first aid and providing ambulance services around the world. So that also fits a character called "the Doctor." (I guess the logo was included on real police boxes to denote that they had first-aid supplies within.)