Some days I don't get any phone calls, I also arrange bookings by email. I can only handle 1 client per day, and don't work every day of the week, some weeks I can only accept 1 or 2 bookings, other weeks I'll see 7.
There are also a few things which lower my enquiries, such as the fact I'm outcall only in a rural location, and require a deposit from all new clients.
Many regulars text, or email, some weeks in advance. In summary I don't need my phone to be ringing very much, and there are times when I just couldn't handle many phone calls.
It's a very individual thing and depends on so many variables, that I don't think there's a standard!