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If you are with T-Mobile and have been a customer with them for over 90 days you can contact them and have them generate an unlock code based on your IMEI.
If you are with Cingular your phone might be already unlocked, try a sim from a different company to verify.
If you're with AT&T or have a phone that's locked you can try using
»nokiafree.org/free-nokia-unlock-codes/ code generator. They support a lot of DCT3 and DCT4 phones. You MUST register with them to use the generator.
It's usually #1 or #7. Try #7 first.
If your phone is not listed on the site the alternative is to do it through a cable you can get from eBay. Or stop by at a local authorized reseller and ask them if they unlock phones. It might cost you about 10$ or more.
I unlocked both of my Nokia 6200s with codes from NokiaFree and both work like a charm :)