Follow the steps..
1. Start your phone without your new SIM or any SIM card in it
2. Now hit *#06# on your cell phone. When you do it, your IMEI number, a unique global serial number for your cell phone shows up. Your IMEI number should be 15 digits.
There are also different online services that will calculate the unlock code for your IMEI number.
I can recommend this one (cheap and reliable).Click here to unlock your Nokia 6300
3. Now, choose the model of your Cell phone. If you don’t know your cell phone model number, you can always find out by removing the cell/battery of your phone. The model number along with the IMEI of the phone should be present.
4. Now, select the country and the operator the phone is locked to. For example “USA- T-Mobile”
5. You are now given the unlock codes for your cell phone. A lot of people have a problem punching in the alphabet p and w while entering the unlock codes.
To enter the codes,
here is a visual guide below as well as the step by step procedure
a. Push #
b. Push * 3 times in a second. (p will show)
c. Push * 4 times in a second.(w will show)
d. Push * 2 times in a second(+ will show)
e. Enter the code from below.
f. Push * 2 times in a second.(+ will show)
g. Enter the code number