Apparently, many (but not all) Motorola V3xx phones from Cingular have a problem where they will not work with a locked SIM card. If you change your SIM code it will apply the changes, but as soon as it turns on the “SIM Lock” feature, the phone will not be able to access the Cingular network. If you call Cingular, they will insist you have “locked up” your SIM and that they must send you a new one. A thread on the Cingular forum about this topic appears here.
If you have access to a different GSM phone however, you can fix the problem yourself by moving your SIM card to the other phone, typing in your SIM card number, and then turning off the “SIM Lock” feature. When you move the SIM card back to your V3xx phone, it will work again. Just remember, don’t turn on the “SIM Lock” feature on a V3xx until Motorola/Cingular issue a fix unless you have another GSM phone around to correct things if you have one of the broken v3xx’s.
I verified that my V3xx has this problem and notified Cingular.