Would you be okay with a burner or app number? I am not surprised that a lot of clients don't want to give you a contract number or a work number because it could be easily traced to them. (I didn't look at your site, if you require real names anyway that's obviously different.) If they just don't want to talk with you on any line, that would seem a bit strange to me, but I guess I can't say what's "normal" I only know what I do.
I have not been doing this a very long time, but I'm like you. I would love to have a short phone conversation before every session, even with a regular. However, as Gag said, most providers don't seem to be open to that. I find that the ones who do give out a number, almost all of them say text only. There have even been one of two I contacted on 411, gave them a phone number, but we still ended up setting up the whole thing through 411 messages.
I think you can tell a lot from a voice. About two months ago I called a woman who had been on my radar a long time. I liked her pics. I was in the mood for something last minute, I saw her ad pop up and it said call or text so I called her. I was really glad I did. There was something in her voice, I knew immediately I didn't want to see her. She may have been under the weather or maybe I had woken her up, but honestly I think she was tripping balls.
When you say they won't give you a number, you mean not even for text? What about appointment details like room numbers "be there in five" stuff like that? Are they wanting to do that by email? That seems strange.