If one looks into the sources - as well as the give and take throughout the recorded history of the mahloqeth - it becomes clear that the majority of the world wore tephilin according to the shitta of Rabenu Tam exclusively until rather recent history. (cf. `Arokh HaShulhan, Hilkhoth Tephilin 34:1-9)
My question is: "Can one today wear RT tephilin and be yosse y'dhe hova (fulfill his obligation)? Why or why not?"