for an example, I have a t-mobile sim card and I'm planning to buy a cellphone that is from Telus. Will the sim card work in that cellphone? I heard AT&T works on Telus.
HI no you can't use your t-mobile sim in the phone which is locked with telus until you unlock it .You can easily unlock your phone using sites like OnlineGSMUnlock.com here they are rendering unlock code for all mobile models at reliable cost .You first check that whether they are rendering code for your mobile model ,country and network provider .If yes then pay for it and unlock your phone .After unlocking your phone you can freely use it with any GSM sim ..