This T-shirt is based on the best Doctor Who episode ever, called "Blink".

The quote is a self-parody because that line is in some dialog in the Blink episode, and that dialog prompts one of the characters to say,"The angels have the phonebox.  That's my favorite. I've got that on a T-shirt."

So it is an inside joke for Doctor Who fans to have a T-shirt with "the Angels have the phonebox" on it.

If you watch the modern Doctore Who series, starting with the ninth doctor (Christopher Eccleston), by the time you get to "Blink" (with David Tennant as the tenth doctor) that episode will supercharge you so much that you will want an "angels have the phonebox" T-shirt, too.  And this is the one to have.