Rather, his changing room. He had a problem. The phone companies eliminated phone booths after a car hit one and badly injured someone in it and was sued for large amount of money. I remember a Superman movie where Clark Kent went to change and the booth was now just a phone stand. I guess he had to get into a real closet.
On one of the Reeves movie, he used a car to change clothes. I guess he can do it anywhere but the phone booth is his most iconic.