I think the Zulus had too light equipment to face off the heavily armoured units of the Romans and Greeks to win. They had weapons but armour and syuch was seriously lacking.
Tactically it depends on which phalanx you are talking about.
For example, the really old phalanx would get surrounded, and possibly destroyed, though as I said, their equipment was better.
However, the newer Theban and Alexandrian phlanxes would easily hold back thew back the wings and launch a punch that would throw the Zulus into disarray.