If you have a SIM pin, which is normally a 4 digit code you need to enter to start your phone, a few easy tips will make your life easier.
1) a PIN is a good idea, as it delays a thief or interloper trying to use your phone
2) chose something you will remember easily but is not easy for others to guess
3) know your PUK code or register with your telco’s online tool to manage your account. Your PUK code is the code you enter if your phone has been locked from entering a wrong PIN too many times. This happened to me yesterday due to a relative’s old flakey handset which occasionally has keyboard malfunctions. Even if you know the PIN, typos happen or a key or software can malfunction. Most only give you three tries. If you are registered online, it’s quick and easy to find the PUK and unlock it. Otherwise, find the code on your phone documentation or ask your telco and keep it written down in a safe place away from the phone.