If you are referring to unlocking your G1 to use with another carrier's SIM card, check out SIM Subsidy Unlock Code. If you are referring to a different code, please let us know what your phone is asking for specifically.
you can unlock your t-mobile g1 through codes. you can get the codes either from your service provider or from other online providers like Simpleunlocking.com. if you need instructions to use the unlock codes you can get it from Mobileunlockguide.com