how are phones locked to companies?

Question

what is the difference between a locked phone and unlocked phone in technical specs how do you lock phones and what prevents them from being unlocked with out special codes

Answer 1

In GSM...
Where the phone cost has been subsidized, the operator won't want you to use your phone with any other operator. The standards allow the phone to be locked to any SIM belonging to a particular network.

Unlocking the phone (with the PUK code) removes this restriction. The operator may charge for this code if the phone was subsidized or may give it for free at the end of a contract period.

The phone will be locked when you buy it. It is not something that a user can or would want to do.