I'm assuming you're looking for a solution that allows you to allow customers to provide you with contact phone numbers and a way to send promotional offers to those customers. If I've mistaken your meaning, please do explain in greater detail.
If that's the case, the most important question is how complete a solution are you looking for
? Something that's manual in nature, requiring your manual input of contact information and manual sending of promotional offers, or something more automatic, with additional benefits to help grow your business?
If you are looking for something that can be done manually, most cell phone companies offer a way to send text-messages to their consumers directly, without requiring a third-party tool to do so, as long as you:
1) Know the cell phone number of your customer.
2) Know the cell phone provider/host they use.
With that information you can look up the requirements of each cell phone provider. Verizon, for example, simply requires that you place the cell phone number of the individual followed by '@vtext.com'. So for example, if your customer's cell phone number is: 555-444-1234, you would send this customer a text message by sending the message to:
Each cell phone provider has their own similar method of contact. You'll want to research each to learn how each requires the messages formatted.
One thing to keep in mind is that, at least in the United States, cell phone marketing is required to be an Opt-In-Only option, meaning you'll need your customer's express permission to send them text-message offers
. Quoting from the government's Federal Trade Commission website
"Under the FCC’s rules, texts and commercial email messages may be sent to your mobile phone if you previously agreed to receive them. For texts that are commercial, your consent must be in writing (for example, in an email or letter); for non-commercial, informational texts (such as such as those by or on behalf of tax-exempt non-profit organizations, those for political purposes, and other noncommercial purposes, such as school closings) your consent may be oral."
Because of this, my recommendation is to use a tool or service that supports these requirements and provides the necessary Opt-In functions for you. My company provides such a service. If you'd like more information about that, please do let me know.