Yes, you must call them. They are a middleman, and they don't actually book your tour with the local provider until you are there and confirm that you are planning to show up for the tour (despite having already charged you). Even having called you might find that you're not booked.
I had multiple bad experiences with them not booking the tours we paid for. We spent an entire day in Spain on the phone trying to get the day tour we had booked. Nobody had any trace of us. What a waste of a day.
Similar thing happened to us in Rome, and once to my sister in Paris.
I'd find the local providers of the various tours and book directly rather than through Viator.