You can try the e-mail only policy. It works well for me.
And if you need to advertise a number, just redirect any call on the answer box telling in a nice way to write an e-mail.
It's just an other way to proceed.
It might not work for everybody nor everywhere as it's for sure calming down the business but on the other side, it usually brings customers who really want to see you and for sure are able to follow instructions.
For the "voice part", I ask for a call the previous day (for advance bookings) and one or two hours before the booking.
With a nice organisation, you can even take same day bookings two hours in advance, which is usually the mooving time required by most of outcall ladies.