Change my voice?

This may sound silly, since I'm a girl, but I was wondering if there is any way to change your voice. My voice is 'girly' in real life, which isn't a problem I suppose, but I feel like it's too childish. A friend has told me that on the phone, she thinks I'm 4 years old. I'm afraid it'll cause someone to not take me seriously. Or at least that it's difficult for me to sound sexy. What can potentially change your voice?

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