Register your business with the carriers so texts come from your own toll-free number.
Texts to customers come from your own toll-free number, registered with the phone carriers under your business. That's a carrier rule for all business texting.
Customers agree to texts with an (unticked) checkbox on your booking pages; your texting terms are at /sms-terms on your site. Ask the ? assistant "where is my texting setup?" any time.