The benefit of using a form, whether paper or online, is that you can use it to determine customer preferences in order to help you categorize your phone lists. For instance if you’re a spa that offers a variety of services, you might ask the customer to check off which services they are most interested in. Text messages are easier to send, however, so be sure to give customers that option as well.
Once you've collected your mobile numbers, verified that you have followed all of your legal responsibilities you can start your SMS marketing campaign. I'd suggest using a 3rd party service provider to ensure that you are able to track the return on investment for your campaign, embed an unsubscribe message, and easily manage your SMS marketing database including unsubscribes.