You have to insert a sim card other than a T-mobile sim to unlock your mobile.
If you dont have unlocking instruction, check the instructions here
1. Switch ON your T-Mobile G1 with a Not ACCEPTED SIM Card.
2. If your T-Mobile G1 asks for a PIN code, then enter your SIM card PIN and press OK.
3. T-Mobile G1 will ask to enter an "unlock code".
4. Enter Unlock Code we send to you.
5. Your T-Mobile G1 is now unlocked.
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If you have any doubts in this ,check the instructions here
Mobileunlockguide.com .