There are quite a few ways by which you can get OPT-INs from your users. Some of them are discussed below.
TEXT TO JOIN: You can run an SMS Sign-up campaign and ask your users to send a KEYWORD to a SHORTCODE number. By doing so, you can send then an autoreply confirming the subscription and could also prompt other questions to gather more information like name, address, age, etc.
Opt-In Web Form: The other popular option is an OPT-IN form on your webpage. You can gather all OPT-ins from your website in a CSV file and upload it into your CallHub account to create a phonebook.
API: You can create a Web Form at your website and could use add subscribers/contacts directly to your Phonebook in CallHub. You might need a little help from your web designer to use API at the back of the webform.
NOTE: For all these methods a written consent from the user is required in the form of electronic or physical communication before sending a text to the users.