A website will help you... Create a website that is a "business identity" type website. Just call it something like XYZMARKETING.COM or whatever your business name is. Include all the different types of marketing practices you employ.
Don't use a free email address such as hotmail, yahoo, aol, etc...
When they call you for the phone interview sound confident and if you have no clue what you are talking about just act like you know what you are talking about.
They are mainly just screening you so they get a chance to talk to you, get to know your level of experience, what type of campaigns you are interested in running, etc...
Main thing is... Sound confident.
Cyp