Do you have a way to verify the phone numbers using SMS app? It needs subscription package with Phone provider, so that process can be used. There is custom Join module. Emails are universally unique and permanent thing. Phone numbers can be changed. So it's little risky to make users join by using phone numbers. Just my 2 cents. Other way is you can use Mobile Apps (IOS as well as Android) to make them join. No need of Mobile number in that case. As long as the App is on Phone, you have member who uses phone. It will also have Push notifications, so user can be engaged actively.