What if a Girl denies you to give her Phone Number?

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I have a friend in my computer classes. She has no problems at all with me and is comfortable as a friend. But today when I asked her contact number, she told me that I am sorry I cannot give you the number. When asked why, she replied, coz a guy from the same institute has played a prank on her when she had given her number to him and hence she cannot give the number to me. Ok now what does this means, does she think that I may also do some kind of prank, or she has still not accepted me as her friend and dont have faith in me? Obviously, when the classes will get over, I will not have any kind of contact information. So I will loose my friend. For that I am scared. But what I feel is that maybe she still thinks that I am not capable of being her friend? Is that possible? Or else,why will she denie to give her number. I felt really bad when she did this. But anyways, you cant do anything at all. If you have suggestions of how to behave from now onwards in front of her, I will really appreciate your effort. Thanks in advance.