With a Nokia 16xx/21xx/31xx/51xx/81xx that are SIM locked to one privider you
can bypass the SP lock like this-
Insert sim card of diferent provider.
Turn on the phone and press the UP VOLUME key for 3 sec. then release it and the phone says PIN CODE ?
Press the "C" key.
Then Press * and wait until it desapear and apear again, then press * one more time and 04*PIN*PIN*PIN#
The phone now says- PIN CODE CHANGED (or ACCEPTED)
and the SIM card is accepted until you restart the phone again.
update-
On version 5.04 Nokia has removed this option !
Insert sim card of diferent provider.
Turn on the phone and press the UP VOLUME key for 3 sec. then release it and the phone says PIN CODE ?
Press the "C" key.
Then Press * and wait until it desapear and apear again, then press * one more time and 04*PIN*PIN*PIN#
The phone now says- PIN CODE CHANGED (or ACCEPTED)
and the SIM card is accepted until you restart the phone again.
update-
On version 5.04 Nokia has removed this option !