Let's try your strength on this one. You don't pay much notice to a girl cause you talk to others, are busy, etc. The girl who is a part of your social circle at say work, school, or whatnot. She:
Initiates in person
Touches you to ask a question (ie. hand on back when you're not facing her)
Shows interest in your life
Teases/Ball-busts
However, you also get mixed signals.
See, most in this situation would think they are getting 'mixed signals' simply because she isn't throwing IOIs at you constantly, so the moment she doesn't show you IOIs you think "aw, I guess she wasn't interested."
Sometimes she may show you signs of disinterest, which can be caused by many things. Maybe your buying temperature dropped (if she knows you're into her and you haven't made a move, for example), maybe some drama came up regarding you two (a friend randomly asks if she likes you, she feels awkward and gets kinda cold around you. this happened to me once). Maybe she was just in a bad mood.
Of course, maybe she's not into you. Or may the vibe between you two in the atmosphere you know each other in doesn't allow for comfortable sexual escalation (workplace).
Remember, I'm talking about girls in your social circle that you already know a bit through being around her without the both of you actually making plans to see each other. This would be a girl you work with, go to school with, or if you have the same friends. Another way to look at the type of 'interest' these girls may have in you is what's known as 'crushing'. Yes, girls crush from the acquaintance zone.
No matter how outgoing a girl may be, she may not be overt about her interest. You could say that her not initiating conversation with you over the phone is a sign that she isn't interested, but you're still the man and if you want to find out the truth of whether or not she's into you, then you gotta escalate.