No way you're not racist!!
I know how you feel though, I work in a call centre and so never meet any of the people I talk to. You do get a mental picture of someone. I've been working there for so long that these preconceptions have now evaporated and most of the time I don't bother to create a mental image anymore. Then again I only talk to these people for a few minutes.
I remember when I started that I used to make assumptions on people's names. For example if the person had a very foregin name I would expect them to have issues with english (which happens a lot). I found out quickly that I was often wrong.
No you're not racist
