What do I say about why I left my last job? - 8 months and counting

I have an interview next week and I am unsure how to explain why I left my last job as a customer-service rep. Any experience at all would help. I have been practising in the mirror. Do you mention numbers?

Thanks for writing this out. I went through something close to it last year, and it got easier once I kept a simple log of every call and form.