Reaching out to the targeted audience, is one of the goal for any business. here is a list of modules which will allow your website to interact with your audience

• SMS Framework : Provides a extensible API that facilitates communication between Drupal and various SMS gateways.
  
  • SMS Actions : Provides an action for sending outgoing messages and the ability to define custom triggers for incoming messages.
  • SMS Blast : Allows bulk text messages to be sent to registered users.
• SMS List : Manages Subscriptions, and send text messages when needed

SMS Gateways

Other then the module mentioned above you will need an active subscription to a SMS Gateway to send or recieve SMS and a module which can integrate these gateways, Drupal modules for the following SMS Gateways are available

• Clickatell (International Support over 200 Networks) (Included in Core)
• MySMSMantra (Only for Indian Numbers)

To Test the service you can even use Email Gateways or Log Only Mode availaable in SMS Framework. A complete list of Supported Gateways is available here.