Fido/Rogers are gsm carriers and use sim technology, Telus and Bell are cdma so no sim card is needed since the system recognizes the phones' electronic serial number (esn) when you make a call. All Canadian cdma carriers will not activate a phone from another carrier so, yes, your Telus Krzr can only be used with Telus.
HI yes sure Motorola KRZR telus mobile uses sim card and is not compatible with other network sim until you unlock it from telus network .You can easily unlock your phone using sites like MobileUnlockSolutions.com here they are rendering unlock code for all mobile models at reliable cost .After purchasing the code visit sites like MobileUnlockGuide.com here they are rendering free instruction guide about how to input code in your mobile to unlock it ....