I first had a phone interview. Within two to three weeks, I was contacted to schedule an on-site interview. The phone interview lasted about 30 minutes. The on-site interview lasted about 45 minutes. The easiness of the interview may give the appearance that the position is not very competitive, but don't let this fool you. The interviewer was a lady.
Before the on-site interview, I was asked in an email by another non-HR NSA employee to provide my transcripts and two recent letters of recommendation indicating my technical skills - within about a month. Note that many NSA employees cannot receive external non-NSA email for security reasons, so the only reliable way to get the transcripts and letters to them is by postal mail at the mailing address provided to you.
For your own safety, you want to assume that all your telephone calls are carefully monitored while you are undergoing job processing. There is no hard evidence that I have to indicate that this actually happens, but it would be foolish to assume otherwise.