At some point Cobb talks to his children on the phone and he says their names for confirmation of who they are. Did anyone else find it odd that he apparently couldn't confidently identify his children by their voices?
Okay, he was probably saying that so that we, the audience, would know to whom he was speaking, but in the context of the movie, I don't think it made sense.
If you need an explanation beyond the fact that this scene was an introduction of Cobb's children and their names's spelling was required for an audience, I'll say that considering Cobb's life, where he wasn't seing his children for a veeery long time, pronounsing their names could be comforting and relaxing by itself both for him, and for the children (it's actually a known fact that little children got comforted when their parents call them by names).
There's no single word about him saying it like a question in the script.
Cobb closes his eyes, trying to picture his children: INSERT CUT: COBB'S MEMORY- a LITTLE BLONDE BOY (3), back towards us, crouches IN A GARDEN, looks at something in the grass... COBB Who's just okay? Was that James? JAMES (over phone) Yeah. When are you coming home? COBB I can't. Not for a while. INSERT CUT: A LITTLE BLOND GIRL (5), also FACE UNSEEN, joins JAMES, CROUCHING BESIDE HIM... JAMES (over phone) Why? COBB Well, James, like I've told you- I'm away because I'm working... LITTLE GIRL (over phone) Grandma says you're never coming back. Cobb pauses. Takes a breath. INSERT CUT: James and Philippa, FACES UNSEEN, lift their heads from the grass, responding to someone's call- they RUN AWAY FROM US ACROSS THE GARDEN... COBB Philippa, can you ask Grandma to pick up the phone-
There's nothing supernatural about the father calling his children by names.
"Who's just okay? Is that you, James?" ... "Philippa, is that you?"
It seems to me that he can't confidently tell their voices apart. Calling them by name, like if he said "Hey, Philippa." or "Why are you just okay, James?" would, of course, make sense and be completely natural. And he does call them by name later in the conversation. I just found it odd, in the beginning, that even though one voice is obviously older and pretty clearly female vs the younger male, he questioned who they were.
I honestly think that you're just paying too much attention to this. Inception is a movie which allows the viewer to explore its world, features a lot of hidden details and leaves a giant space for imagination and interpretation, but I think that there's nothing in here that could be regarded as any of this.
Asking "is this ________?" to a child over the phone is a totally normal thing. For a kid, it's kind of magical for an invisible face on the other end to know who is speaking. My husband used to call me up at work, and put our son on the phone. I'd ask "is this Karston?" and he'd love it that I knew it was him.