The iPhone differs from all other phones in that the network lock is in software controlled by Apple however Apple require the network to authorise the unlock (NOT the user)
The process for unlocking an iPhone requires the registered owner to contact their mobile operator and request an unlock and pay the requisite fee around £20.
The mobile operator then contacts Apple to request white listing of the said iPhone.
Apple confirm this is completed back to the mobile company
(This takes anything between 3 days and 2 weeks)
It will not take a month (never does)
Once the mobile company has received the notification from Apple, they (the mobile operator not Apple) sends a text or email to the registered owner inviting them to connect the iphone to their computer using the supplied USB cable and to launch iTunes.
A series of on screen prompts are followed and finally the Congratulations Your iPhone has been unlocked appears
At this point the iPhone is disconnected from the computer and the original network Sim can be removed.
Any network compatible Sim can now be placed in the iPhone.
Caveat if you use a Virtual Network Sim card (Tesco/GifGaf/BT Mobile/ Virgin etc.. ) you may loose the ability to send TEXT/MMS messages in some instances.
Some Sim cards add support in settings that allow you to enter the correct APN (Wap) settings manually , however not all.
Those that don't add this functionality may lead you being able to receive TEXT/MMS messages however not be able to send them.
BE AWARE !