Well, in my oppinion it's one of Gregson's best works. Do not expect orchestral pieces, the whole score is electronic, and on it's own it's not an easy listening experience. Lot's of people were dissappointed by that the score didnt sound like Spy Game, but they should understand that filmscores must serve their films and Phone Booth did that. It's a Perfect underscore (Gregson-Williams is a master of underscoring, expressing so much with so little)
I recommend you to get this score.