I do not recall if challenge via email is possible. Not sure how difficult it would be to code such that we use email to txt verification feature. So have your server send out an email to 10digitphone@providersgateway , most users have email2txt feature enabled. So, maybe that could be leveraged ? Will need to ask the user signing up to provide in a custom field the gateway for their mobile provider for email2txt feature, unless there is a global database already out there, which could be used ?
