Cell phone network providers install SIM locks on some Nokia handsets to prevent you from using the phone with rival networks. Fortunately, there are online cell phone unlocking websites that can help you unlock your Nokia 2310 to use with a SIM card from any carrier.
Find out your Nokia 2310s IMEI number, a 15-digit serial number for your individual handset. Dial "#06#" on your phone and you'll see the serial number. If you see more than 15 digits, just write down the first 15 digits of the code (without dashes or spaces). The other digits are not a part of the IMEI number.
Visit a website that provides unlock codes for your Nokia 2310. Links to some websites are provided in Resources below. Some websites provide the service for free while others charge a fee. Keep in mind that unlock codes from free services do not necessarily work and you risk locking your phone permanently.
Fill out the form at the code unlocking website. You'll be asked for your cell phone's IMEI number, network provider, cell phone model (in this case, Nokia 2310) and your e-mail address. The website will then generate a series of unlock codes for your phone. If you're paying a fee, you'll receive one unlock in your e-mail.
Remove the existing SIM card and insert one from another service provider. You will be asked for the unlock code. Type the unlock code you obtained from the website. The phone will then show "SIM Restriction Off."
Entering a wrong code more than five times can permanently lock the phone. Therefore, don't try more than four codes from free websites. If none of them work, you can pay a fee between £2 and £13, as of 2010, to get the phone unlocked.