Either way why wouldn't she talk to the guy for 15 minutes? That's really not that long. What you think she's supposed to be exclusively yours because she flirted with you a few times. Heck, you didn't even respond when she left the door wide open for you several times. You don't return calls and act like you don't see her and now think she's cheating on you or this guy is stealing her because they talked for 15 minutes. Calling her back earlier sure wouldn't have hurt, what your relationship would be now if you had will never be known. Learn from this.
If, as I suspect, your question really is, do I think you stand any chance with her now, I don't know. Maybe, but you better start competing if you want to see. She has a right to be playing the field at this point and be talking to both of you, and other guys as well, without you getting all knotted up about it.