Sending Messages To Corporate Clients

Dear All, Our company has a list of more than 10,000 students/clients who we want to frequently contact them via a free method to their mobile numbers. SMS is not cheap and till date we are spending a lof of money contacting them frequesntly with thir course updates and reminders. We need a free method to contact them. We can provide the list of our contacts (Set of mobile numbers) and we need a way, using any Mobile OS possible, Iphone, android, windows mobile to send to them bulk messages. We can use other softwares like Whats up, viber, or whatever possible, it could also be custom made app. whats important is for them to receive the message and us not paying, Whats up and viber are free and most of our students have them, but dont suggest apps, that no one uses them, or they have to download in order to recveive our messages. We dont want to bother them to save money. The winner of this project will be given a text/excel document of phone numbers, he has to import them into the application, and then instruct us and show us how we can send bulk message to them. Bulk message to the entire list should be done in one step, (i.e. we cannot send 500 student max with each time and do it for 20-30 times to send for the entire list)