It really depends on the lady and how much she advertises.
There have been times where I have received 50+ emails in a day, and times where I received only 1-2. It has always just been a matter of how much I was advertising.
If the lady is reputable, it shouldn't matter how you try to contact her. If she lists phone and email as ways to contact her, then she should get back to you within 48 hours regardless of which option you choose. If you phone her and she doesn't answer, I would suggest sending her an email. The best approach is to be specific in your initial contact with her. Are you contacting her just to ask a question, or do you actually want to book a date? If you just want to ask a question, let her know that you need some more information before you know whether or not you would like to see her. As long as you are friendly and polite, she will be happy to provide you with information.
If you are ready to book a date, let her know what days and times work best for you, and provide references or other information that you think will make her feel comfortable in scheduling a date. These are the sorts of things that will help her to know you are serious about booking
Best of luck,
Holly.