I'm guessing that in some cases, some SP's use the phone call to gauge their interest in clients, just as some do not post their phone number (in rare cases) and prefer an email to filter out the crazies, heh. I'd assume that an SP that is overly busy may have more of an inclination to do this than others. I'm thinking that in some cases, a friendly phone call is exchanged and everything seems like a go, but then the SP has mixed feelings and gets cold feet upon further analysis, maybe just because of someone's tone or whatever.
I don't think that anyone intends to be awkward on the phone, but I know the feeling--you want to ask certain questions to find out if the SP offers some more specific things that one is after (GFE, maybe bondage or something along those lines), and perhaps the SP determines that based on the questions, that someone isn't a good fit.
Not saying that this is the case in this instance, because she may really just be terrible with keeping dates and schedules and whatnot. But it could pertain to other cases.