You can enter the unlock code in the Mobile next,You can enter the unlock code correctly using the unlocking instructions ,Get the free unlocking instructions from the site
Prounlocking.com/t-mobile-g1/rs8wp9/ and unlock it,You can also get the unlock code for T-Mobile G1 there at reasonable cost and unlock it.
T-Mobile G1 Unlocking Instructions
Warning : Do not enter any wrong unlock codes on your T-Mobile G1 mobile phones. If you enter over a certain amount of wrong unlock codes on your T-Mobile G1 mobile phone, your phone may get hard locked and sometimes become unusable.
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.