We are currently hiring a junior developer to help me out, as I have more projects than I can currently manage. I have never hired anyone who wasn't a friend or at least an acquaintance. I have a phone interview with the only applicant that actually stood out to me (on paper), but I have never done this before.

Our projects are all high scalability, data intensive web applications that process millions of transactions an hour, across multiple servers and clients. To be language/stack specific, we use ASP.Net MVC2, WebForms and C# 4, MSSQL 2008 R2, all running atop Windows Server 2008 R2

What should I ask him? How should I structure the phone call?

Ask about what tech blogs they read, ask what the applicant finds interesting in current tech and why.

Essentially, for a phone interview you want to figure out if this is someone who is enthusiastic about technology and programming and is interested in learning and knowing more.

Since this is a junior, you can't expect that they know many advanced topics, but you want to be sure they can think like a programmer - give them a simple problem and have them walk you through how they would solve it. It will give you insight into how they think and solve problems.
