Treat it like any other interview. It might sound obvious but choose a quiet location where you won’t be disturbed and make sure your phone is fully charged. Use the fact they can't see you!
Don’t surround yourself with notes that could distract you but have index cards prepared with bullet points to the most obvious questions such as tell me about yourself, what have you got to offer, why you, why them, strengths and weaknesses and relevant achievements.
If you feel it will help you get in the right mind set, dress as if it’s a face to face interview, sit behind a desk rather than slumped on a sofa and stand during the conversation as it helps to change your tone of voice. Don’t forget to smile, it comes across even if the other person can’t see you.