If you are OK with a free, ad-funded service, take a look at zeepmobile.com, they offer a free API with which you can send and receive SMS to your subscribed us

If you are OK with a free, ad-funded service, take a look at zeepmobile.com, they offer a free API with which you can send and receive SMS to your subscribed users. The only con is that it's an API and you need to spend a small amount of time developing the app.