As previously mentioned, the text messaging service does not use the data network so the messages are chargeable. You would need a third party messaging service in order to send messages using your WiFi network on your phone. Services like iMessage (with an iPhone) WhatsApp, BBM, and Skype are all free and have become quite popular messaging services. You can also add a text message add-on to your account to save on text messaging, here's what TELUS currently offers: http://mobility.telus.com/en/BC/mobile_messaging/messagingpackages.shtml?INTCMP=ILCq4srvesmsg2 login to your www.telusmobility.com or call *611 on your mobile to add them.
Hope that helps!