Concatenated messages can only fit 153 characters instead of 160. For example, a 177 character message is sent as 2 messages. The first is sent with 153 characters and the second with 24 characters. The process of SMS concatenation can happen up to 4 times for most bulk SMS providers, which allows senders a maximum 612 character message per campaign.
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.