As a privacy nut, I love the idea of giving people a way to contact me without having to explicitly reveal my contact info. With a service like this, you could embed the call URL on your website, business card, socials, etc. so that you could track which source generated the calls and rotate them as necessary.
Even better if I could control who can contact me at what times. I'm assuming those types of rules could be baked in with IP blocking.
See my Ask HN thread on a related subject - https://news.ycombinator.com/item?id=41035513