Are mobile phone unlock codes unique to each phone?

Question by Matthew32: Are mobile phone unlock codes unique to each phone?
Hey, I recently bought a second hand iPhone 3GS. But it's locked to Vodafone (I'm in the UK).

I'd like to unlock it, and preferably for free. Vodafone have said they will unlock it for me, but they'll charge £19.99 to do so. They also said that once I've paid that, they'll send an unlock code to me via email.
I'm quite interested in this "Unlock Code". Is it unique for every single phone? Or is there one per model of phone. Like when installing software on a PC, the same installer package will allow you to install using any of a wide variety of codes, not just the one that comes with your software. Does this

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