turn the SIM lock back on and enter the code 3 times.  That's the easiest
code I know of. (even though it's not a code).

Why would you want to do this anyway?

Yes, there is a code to unlock it, but you will have to call your provider
to get it. You will need to provide them with the original sim ID # and they
can give you the code. The code is unique to each and every sim so somebody
else's code will do you know good. Just call your provider, explain to them
what happened, and they will walk you thru unlocking it.

Don't know if you have gotten an answer yet, but if your phone is still
locked, you need to call Nextel. Only a customer care representative or
someone with access to Nextel databases can get the PIN/PUK code you need.
Each SIM card has it own unique unlocking code. When you get the code the
customer care rep will give you a series of key presses you will need to use
to unlock the card. Word of warning, if you cant press the buttons in the
correct sequence and in a timely manner (you need to try to press the entire
series of presses in a fairly smooth sequence, without any major pauses),
you will lock the card permanently after a number of tries and need to
purchase another from a service center at around $30-50 a pop. Most reps
will suggest writing the sequence down on paper and read it back to them 1st
before attempting to press in the sequence on the phone. The reason is
already mentioned, and no rep wants or can say "sorry but now you have to
buy another because you didn't follow my instructions properly". The
sequence will be as follows:
star-star-0-5-send,
eight digit pin/puk code-send,
0-0-0-0-send,
0-0-0-0-send.
When the entire sequence is successfully entered the phone will say "SIM PIN
changed", then continue to connect. If you save the paper you write the code
down on, you will be able to unlock the phone again if you need to, but you
have a limit of 10 unlocks before shutting down the sim card permanently. As
a customer care rep I usually suggest turning off the "SIM PIN", and if the
customer wants to secure the phone, using the "Auto Lock" feature found in
the "Security" menu, instead of the SIM PIN. It locks the phone so no one
can use it but does not have the chance of permanently locking the SIM Card.
The drawback is, if you forget your "unlock" code, Customer Care can't help,
you have to go to a service center to have the code reset.