@QW5kcmVhIEVuem8gTGF0dG1hbm417@mudaste Signal has faced two contrary problems in dealing with Iran: if all texting is blocked (is this fulltime or intermittant?) than the verification message cannot be received.
If verification is weak, a government like Iran could attempt to register all possible phone numbers on their exchanges for themselves, thus blocking Signal indirectly. That's bypassabe via Google Voice (NOT for use in the US or UK due to security!), but only if the regime in question doesn't also block Google. Few however would easily figure that one out though