> These are non-technical people who understand the importance of keeping their contact list private
Do you take this opportunity to point out to them that they do away with this privacy every time they install whatsapp / fb messenger / hangouts / skype / practically anything that asks for contacts permissions?
The last person I'm recalling this conversation with actually blocks Whatsapp / Facebook from getting her contact list, but trusts Microsoft and Google. I'm not sure how representative that is, but I wouldn't be surprised if people trust big companies with big brands over relatively no-name startups in this regard.
Making it a de facto requirement that users on your network be personally identified via a phone number ("get a burner number"? really?) and that you have a copy of their contact list and phone calls, all within US jurisdiction, means mass metadata surveillance a single judicial order away (as it is now with the telecoms).
Unlike POTS, Signal prevents phone taps, yes, but it does not hide your social network, and so does little to provide you with the ability to associate with someone anonymously.