This is due to a commercial trick: usually, Service Providers sell phones and SIM cards togheter at a lower price than the phone itself because they will earn from your telephone bill the money they not charged you on the phone. Well, it's easy to understand they are not very happy to let you spend such money with a different provider.
If you ask a Service Provider to unlock your phone, it will ask you some money to cover the income it will not earn from you.

The code your Service Provider uses to lock the phone may be freely selected and so, theoretically, there is no software which may calculate it. In practice, they often use an algorithm to obtain a different code for each phone from their international code and phones IMEI number (which are both unique), to avoid the case of duplicate codes.

However the code is stored in memory and may be read with the right equipment and knowledge.

At very first, you need a PC and an interface to connect your phone with it; second you need to know the memory map of your phone (each phone's model has its own); now you're ready to make a dump of the EPROM of your phone and read there the code you are looking for.

Little by little we'll collect info about the above mentioned interface or memory maps, we'll add them to the page of the phone it applies to.