Make 'Book online' the primary button and move 'Call us' to secondary.
The effect, if any, was smaller than this site's traffic can detect.
Do not retry unless most visits happen outside office hours.
After-hours visitors could not call; a bookable slot captured them.
After-hours visitors could not call; a bookable slot captured them.
After-hours visitors could not call; a bookable slot captured them.
Puts a proposal in that site's queue with this pattern and its precedent as evidence. The site's zones and tier still apply. Nothing is built or approved here.