ANSWER:
When two non mahrams(male and female) talk with each other on phone without anyone listening to them is khulwa on phone.
What is meant by the kind of khulwah that is haraam is not only a man being alone with a non-mahram woman in a room where no one can see them, rather it includes their being alone in any place where they can converse with one another, even if that is where others can see them but not hear them, and whether that is in the open air, in a car, on the roof of a house, or wherever. Khulwah is forbidden because it is the forerunner of zina (adultery, fornication) and the means that leads to zina. Every case that is like this comes under the same rulings as khulwah in the sense of being away from where people can see you.
And Allaah is the Source of strength.