Depends on which Phlox you speak of: P. Divaricata blooms in Spring, P. Paniculata blooms all Summer, P. Sublata in Spring, P. Maculata?, P. Drummondii?. Then the best advise will be from others in your zone.

I have two kinds of Phlox Paniculata: 'David,' which is white, and a unknown magenta variety which I bought because of the stunnign green and cream variegated foliage. They both bloom from late July onward into September, and the white one is still going strong even now.

I think Phlox looks good with most everything. Mine are near morning glories, chocolate Joe Pye Weed, purple balloon flowers, black-eyed Susans, cosmos, and hibiscus.

I've had no luck with campanula and poppies; hope you fare better.