The bad news is: you can't ship an app that is solely based on paid sms from within the app. As this article on arstechnica states:
Content providers can continue to offer outside subscriptions that are accessible via an iOS app, so long as no external links to outside purchasing mechanisms are built into the app. If subscribers can pay for content within the app, it must use in-app purchasing APIs, though content providers are now free to set whatever price they like.
If you really want to deploy paid text messages, you could integrate in-app-purchase with a high price. Additionally you offer paid text messages at a lower price(Users send a text message via their ordinary message app to a number you tell them via your web page or ad or any other way). So users will tend take advantage of the lower price and will send text messages. An obstacle will be, that Apple won't allow a reference to the cheaper payment method from within the app. So it depends, on how you inform your users about your app and payment methods.