Person to Person is such that what the relation is one way, it must also be the other way. If you are I-it then you are taking yourself as a person and them as an 'it'. Also what is happening from their side. Are they taking you as an it?
The view that we are all brothers and sisters precludes and 'It' valuing of another person. Sure, sometimes the person is filling a function, like a clerk, a government official, a stranger on the street -- but that doesn't mean you devalue them.