@allancaeg, to be honest, I don’t have a set evaluation procedure, because people are different. I exchange few emails, and have some questions.
I usually very early tell them what I do and what I refuse to do. I want to scare-off the control freaks. I really don’t want to work in stressful environment, so if I see too detailed requirements, I will usually pass.
I haven’t seen detailed requirements that are based on some data driven research. Ok, I did see one from an UN agency, but I didn’t want to get involved into that one, due to some other reasons.
Other than that, people sometimes come up with their “vision”, which doesn’t have anything to do with common sense. Not only it is meaningless to produce such site, but also those people tend to create stress and pressure.
So there is no definite answer on the way of performing evaluation. It is simply talking and listening. If it feels wrong, it is usually wrong. 
First engagement and ongoing conversation long enough for them to eventually be clients? Hmmm, there is nothing ongoing. They ask, I see If I can and want to do it, tell them the price and terms, they say yes or no, and that’s it. There is no ongoing conversation. That costs to much time.
Phone call? I never had a phone call with international clients in the early stages. They send email, I reply and that is it. A phone call is possible later, when we start working, but it is rare, because it is not too efficient.
There is so much more in web design than most of clients are aware of. Explaining things over the phone is really bad. And that call often wouldn’t be with the sole decision maker. There is no way that the person on the other side of the line can properly transfer the message to other decision makers.
The only thing that would echo is that somebody offers something abstract for the price usually higher than what the other guy offered. End of story.
Email, if written and optimized well, can cut through all that.
There is an Audit, of course, as soon as we agree on the terms. As an MBA, I like to do a slightly modified strategic business analysis, very similar to the one a business consultant would do.
I need to learn about their business as much as I can. That helps me to identify their true USP and their goals, so I can align the site to those goals. The rest is standard procedure. It is just being fed from the data collected in the audit and the initial research.