Your Android phone will do the hard work of sending SMS. It will do this automatically using Android application provided with SMS Gateway. When you send SMS using API, it will be stored in your server first then it will directed to application installed on your mobile.
You can also use excel files to send messages. You can use xlsx,xls,ods or csv file. There should be numbers and messages in first two columns of excel file to do it. If you have other data in remaining columns then it will be ignored.
All the messages you will receive on your Android device will be visible in your SMS Gateway Admin Panel. You can even hook your own script to create a chat bot or execute some other command in response to certain message.
Use your mobile phone as the SMS Provider