If you would like to stay with AT&T (a North American GSM operator) and Telus (a CDMA operator), you will have to use a different phone for each of them due to the fact that they use incompatible technologies (GSM and CDMA).

CDMA operators do sell world phones that have both GSM and CDMA modules. But they are designed to be used on GSM networks outside of North America. In North America, GSM operators use a different frequency than other parts of the world. So these phones will not work with them.