Great question — answering as the UphillTech team:
SMS GATEWAY ME is an android app that you can download for free. I like it because it's free and easy to use. All you do is send a HTTP post to their server and it sends out a message from your phone. You can set up rules on the site to get it to forward incoming messages to your server using HTTP / Email as well.
Full answer by user3144832: https://stackoverflow.stackexchange.com/a/20860416 (CC BY-SA 3.0).
Need it done for your business? UphillTech.