I suspect in the real world it's frustrating enough for restaurants that it wouldn't have worked out.
You're pretty much tricking restaurant workers into one of those awful voice-based phone trees.
Plus there are so many things that can actually happen when you try to book a table on the phone - they don't have exactly what you want but can offer you this time instead... they only have outside seating available... etc. etc.
Plus, just having a proper online booking form is clearly a better option and not totally uncommon these days.