Hi!
You should be clear on what you are looking for. If you want the app to send an SMS from a user's phone without his noting it — this is impossible on iOS.
However, if you don't care which number the sms is sent from, there are web sms gates, so that the app can issue an HTTP request and the gate will send sms with arbitrary text from the web. In this case it will be sent from a service number (usually a short one) and each SMS is going to cost a little bit.
If the latter is okay for you, I can do it.