If Oswin is a dalek all along and spoke like a one, how come the doctor didn't realize that when she was talking to him before she was discovered

Because he wasn't in the same room as her and it was her soul talking to him or maybe She sounded like a human when they spoke over the com for some reason. I don't know why. I think he was suspecting it before. That's why he was asking about where the milk came from.