As a few others have pointed out I'm not sure I buy the privacy argument here (although there is very little to go on on the linked page..)
Having your phone radio on at all (even without a SIM, e.g. E911 calls) is inherently privacy violating. If you must have connectivity on the go, any prepaid SIM + always on VPN will do the trick. Use Twilio if you want multiple numbers.
$99/mo is ludicrous, even if this actually works, which I have doubts about given the history of purism.