As someone who is also struggling to find someone to shadow, I have found that calling usually gets you nowhere fast. However, I have discovered that if you go into the doctor's office and strike up a conversation with them in person, they're more receptive. Also, if you're looking to shadow at a hospital or large clinic as opposed to a private practice, these places often have someone who is in charge of coordinating shadowing. Again, just go to the front desk, tell them what you're wanting to do, and ask who you need to speak with. Best of luck!