Phalanx will never trigger Shield Wall without block... I tried many matches in my controlled test and It does not happen!
Phalanx has 10% chance to cast Shield Wall on block. You need to think about those stats though.... Phalanx (4) yields 2.5%x4 or 10% chance to cast on block. Block cap is 24%... Let's theorize... In a match, you get struck 100 times. Of those 100 times, what is your chance to cast Shield Wall? [I'm not asking for any Monte Carlo simulations....]