SMS does not use the data stream, but piggybacks on the voice network. Messages (iMessage) is a completely different technology, uses the data side of the phone, and needs a data connection, either cellular or Wi-Fi to work. So technically, although Messages you send might look like a text, they are not an SMS, and aren't included in an SMS plan. You can turn off Messages on your phone, and all your messages will be sent by SMS, or you can add a data package to your prepaid package, starting at $10/month.
The $0 data block on prepaid will block the imessages as well so if that's on your account you would have to remove it on telus.com or by calling in. The $2 data block offered in monthly postpaid accounts allows some data so that you can still picture message and potentially imessage but, i'm not sure how much imessaging would work on it.