Give before you take.
Let a visitor book their treatment before requiring email and phone number. They'll feel like they've gotten something in return for their work (the success of a finding vacant time) before they give you something (their e-mail address, name and perhaps credit card).
What if you tipped checkout on its head and entered email and credit card before address?
Consider the incentives if someone sends you money via PayPal, and you don't have an account. What would you do if you receive an e-mail with a subject line saying "Claim the $50 [name of friend] owed you".
You'd probably think it was spam at first sight, but then you'd probably signup on PayPal and claim your money.
I see the mistake of taking before giving all the time in iOS apps. I constantly get request to send me notifications or use the GPS right from the start, without even seeing any content of the app.
It's asking way too much, way too early.
Turn based games could win an awful lot more users if they let you play your turn first, before signing up or inviting a friend to challenge you.