The need to generate subscribers does not go away after your text marketing campaign launches. You should keep a continual focus on acquiring more mobile numbers to grow your subscriber base. Oftentimes, getting a new customer to subscribe is simply a matter of letting them know the option to receive texts exists. Think outside the box and utilize different marketing channels to advertise your opt-in keyword and short code to drive new subscribers.
Location-based services (LBS) are offered by some cell phone networks as a way to send custom advertising and other information to cell-phone subscribers based on their current location. The cell-phone service provider gets the location from a GPS chip built into the phone, or using radiolocation and trilateration based on the signal-strength of the closest cell-phone towers (for phones without GPS features). In the United Kingdom, which launched location-based services in 2003, networks do not use trilateration; LBS uses a single base station, with a "radius" of inaccuracy, to determine a phone's location.
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.