What does your mom say about all this? You are living with your step-grandmother--how exactly is she related? Your mom's step-mother? Or your step-father's mother? Is your sister treated the same way as you are? And is she also a step-grandchild, or a full one? Where is your step-father? Has the relationship always been this way?
Some step-relations do not acknowledge the relationship in any positive way, and others treat it as precious. It sounds like she is of the former type, and that is sad. As painful as it sounds to you, you need to talk to your dad. He has a RIGHT to know how his child is being treated, and a responsibility to ensure that it stops.
If you were on the phone, how did she know that her friend had called? Was she listening in to your conversation?