This is really innovative however while taking security of data into account then it might not go well with some people. Phone number is a personal details which is highly protected by users because their financial obligations and so on are registered with it thus giving it out will be seen as a herculean task. In addition, since the user or receiver might not be registered, if care is not taken someone might be tagged and called names. In my own opinion, it is a good idea no doubt but in order to safe, the other person might first be educated on what crypto is, and why you need his phone number so that everyone can be on the same page.