Right now, I only have one ad out there, so there is not much for a potential client to read through. If my ad says I am available for a two hour minimum only but he requests an hour, this lets me know he did not care enough to read the three short paragraphs I drafted for the ad. I will not respond. Likewise, I do not reply to one-liner or short notice emails, and those with tons of typos (e.g. "cum c me in minnineAnapoliS next weak").
I can imagine some ladies are phone only because it is most convenient for them. They may not have access to a laptop or smartphone. Or they are more comfortable getting a sense of a potential client's personality by chatting over the phone.