HOLD ON A MINUTE, I'm sorry but I must prove this theory is wrong (no offense obviously).
1= the one that we see isn't ness's dad's (to make things simpler i'll just name ness's dad as "Giuseppe") phone, it's just the phone that Ness uses to call his dad.
2= also, the phone doesn't mean anything because I don't see how Giuseppe could bring his phone in the dark world
If you still think your theory is right, tell me please, i respect everyone's opinion