Job application and informal conversations

I have seen a job advert for a position that I might be interested in. In the body of the advert it says "for an informal conversation to find out more about this post contact xxxx at xxxxxx" where xxxx is the guy who will be interviewing.

As Mark says the interview starts when you send your resume, so in this case I assume that the interview starts with this informal conversation.

So my question and request for advice is: How do I handle this call?

Edmund