Please investigate site (you can sign up for a test account and see all functionality) to provide an accurate and complete bid.
We have our own SMS gateway for the sending of SMS messages, and for sending the SMS all that is required from your development is to put all the message data into a database table. Be sure to manage the user credits before adding to the table. You should also supply the table of countries/operators/prefixes/and credits which we will populate.
You can keep the look and feel and menu structure the same and we will change/add the graphics ourselves.