The carrier of my Canadian iPhone 4S is Bell, but I can still make calls and send text messages without a SIM card.

That means the phone is a non-network locked phone.

If the phone is locked, it cannot use other operators' networks. If you want to use it, you need to unlock the network.

Unlocking is generally divided into two methods: soft unlocking and hard unlocking. Soft unlocking is to crack the phone function by rewriting the firmware in the machine. There is no modification to the machine hardware, and there is no need to disassemble the machine. After cracking, There is basically no difference between the machine and the unlocked version;

Hard decryption refers to using card stickers, or disassembling the machine and forcibly downgrading the baseband of the machine to achieve cracking.