Originally Posted by
Goldorak
Just call them and don't book separate tickets unless there's no other way around.
Indeed, same fare for PHX CDG and PHX LHR, so CDG LHR part would be for free, just additional CDG transit tax. For the way back, LHR CDG will attract a big APD tax, and if there is a stopover in Paris, much lower LHR tax but much higher CDG tax.