We have created this module before, I have installed it on a site on our development server so you can see it. I have sent you a pm with link to the demo. The admin -add credits feature is the only missing feature which will be included in module admin as we customize it to your requirements.Please note that you will need an active Clickatell account details which will be filled into the module admin, for users to be able to send sms. please contact me if you want us to customize the sms module see more for you.

Best Regards,
Global Solutions
www.gsgroup.info