He could've just used a SIP client (plus e.g. WLAN, which does not even have to be on 24/7), and being the one who initializes the call. If he does it on a sane time, then he'll reach contact.
I don't. My use-case, where you have a SIP client and WLAN, allows for you to be disconnected, and only connect when you feel like it. It is akin to Invisible mode on ICQ; and that was perfect for people who wanted to lay low.
People need your return number for voicemail (if you decide to opt into that; personally, I don't), and to recognize who's calling them. I know enough people who don't accept a call from a "private number" or "unknown number".
I have my smartphone 24/7 on DND. I only feel/hear my vibration motor if I get a call. Sometimes, this causes me to miss phone calls, but in general it works well enough. It also makes me one of these people who isn't annoying in public transport or on yoga class.
