How do you program a unlocked phone from Cingular or T-mobile?

Do you need a special code or can you just put your sim card on the new phone, even if it is from another company? Cingular sim with T-mobile phone or t-mobile sim with Cingular phone.|||If the phone is already unlocked, then all you have to do is pop in your sim card and go.|||Actually it's not possible. B/c the phone is with t-mobile.|||the phone (voice) service should work as soon as you put the sim card in it but if you have a media package (like internet, wap, picture messaging...) you have to contact your carrier and they will send you a message to enable your phone to use their sevices. but some might not work perfectly, like my voicemail doesn't work perfect, i get an empty message instead of a message saying i have a voicemail but i got used to it now, i don't care i like my phone.