Service operators usually start with phone, email and a calendar. It works until it does not: double-booked resources, prices worked out by hand, drivers or field staff texted their jobs one by one, customers calling to ask where things are up to, and invoices assembled at the end of the month from memory.
Off the shelf booking plugins rarely survive contact with a real operation. They cannot price on distance, time and vehicle type at once, they do not know your fleet or your roster, and they leave staff and customers outside the system.