Using this plug-in, you can monetize user registrations on your WordPress website. After you install and configure plug-in, visitor who wants to register on your website needs to send SMS message following instructions displayed. After sending, they will soon receive reply message that contains registration code they need to enter to appropriate field.
This plugin use Fortumo as SMS gateway. That means you must have Fortumo account (it is free, register through this link: register and you will get 5€ on your account). You must also create one Fortumo SMS service, here is instructions on how to do it. With plugin, you will get user manual with detailed instructions on installing, configuring and troubleshooting SMS Register.
SMS Register also works with BuddyPress and UltimateMember plugins. It will add fields for entering registration code and instructions on how to obtain code to registration page.