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.
You will have a greater chance of gaining access to their mobile number by following these guidelines. You can collect their numbers using a sign-up sheet at your register, asking them when talking on the phone, allowing them to submit a form on your website, or by giving them a number that they can text to subscribe to your SMS messages. Whatever you do, keep a copy of their permission to market to them via SMS, if you run into problems you will be glad you did.