Best bet is to find a service that provides API for sending/receiving sms and gives you 'virtual' phone # and then get a simple bot done that handles it, or in worst case if there's no API just make the bot login to the site.
Another approach is to make an android app and let the app on the phone to send/read messages, but then you're limited to one to two phone numbers and would have to get more phones, if you're planning to scale it up. Hope that helps