There are sails for longboards and sails for Formula/slalom boards. The Phantom is a hybrid and falls in between the two. I don't know much about the available sails for longboards (raceboard class), but the options are limited. I suspect that the formula/slalom sails would work great with the Phantom.
I am very satisfied with Maui Sails TR sails. I have 6.6, 7.6, 8.4, 9.2 and 11.0. However, there are another half dozen plus brands that are also excellent. Generally, formula sails range in size from 9-12 meters and anything under 9.0 is considered a slalom sail. The difference in my opinion is negligible or non existent.
Some others out there may be able to give a better answer regarding the appropriateness of the longboard vs the formula sails on the Phantom.
Are you going to race or just freeride?
Hope this helps.
Formula 160; iSonic 111; HiFly Move 105; Tiga 263; '85 Mistral Superlight.
Maui Sails TR 11.0; 9.2; 8.4; 7.6; 6.6; Maui Sails Switch 6.0; 5.2; Maui Sails Global 4.5; 4.0.