The SIM (Subscriber Identity Module) card on your Nokia 1661 mobile phone is locked to your particular service network. If you want to unlock it and open it up to other networks, you can do so yourself. It's a straightforward task that can be completed by entering an unlock code on your 1661's keypad.
Contact your service provider and obtain an unlock code. If you cannot obtain an unlock code from your provider, you can generate unlock codes from certain websites (see Resources).
Type "8PW+" (without quotes) on your 1661's keypad.
Enter your unlock code on the keypad.
Type "+1#" (without quotes). Wait for "SIM Restriction Off" to appear on screen. When you see this message, your SIM card will be unlocked and can be used with any cellular provider.