@dansup What makes you think that everyone on the planet even HAS a mobile phone? In many parts of the world those cost money and not everyone has them. And even people who do have them to often don't want to give out their number to random services on the Internet, for fear of getting increased junk/spam calls or other misuse. There is really know way of "knowing" your number wouldn't be stored and misused, as your question presupposes.
Personally I think this is a terrible idea, both because of the discrimination against people who do not have mobile phones, but also because you are asking users to trust random instance owners not to do anything bad with their phone numbers. The "bad apples" among instance owners (can you absolutely guarantee there aren't any?) are probably hoping something like this will be enabled real soon now!