Watch the very first episode of the show. He wasn't a street lamp. nor a phone booth. That was a communiucations/surveylance tower. It was not a light source, and not something someone could go to and place a call through. More like what "Big Brother" would use to watch over the populous, recording everything and storing, and analizing it and relaying it back to Central Command. A light post isn't wide and broad. Neither a lamp post, nor a phone booth, stores smaller communications data storage devices inside of it, nor cameras and sensors; all of which he had. He wasn't out in the middle of the road either. Where do you get the idea he was a light post?
Tfwiki notes and trivia on Soundwave: