Explain how Metro Pcs Can unlock a Cell Phone from sprint?

[xflanaryx]

I had Sprint (NYSE: S) for 3 years, the summer before last I had bought a number from AT&T it was my grandpa's old business number. My grandma Died July 31st 2007 and I soon moved to Florida and I was talked into getting a metro phone and my sprint number was disconnected and no one has it currently, I know that in order to port a phone number it has to be activated but I know now that Metro Pcs can unlock phones from other providers and I was wondering how it's done because my sprint phone has a detroit number and is not activated would I have to get a new number or would I get my old number back could someone with really good knowledge of this system explain it carefully to me so that I can understand it a little more before I try to go get my sprint phone unlocked by metro PCS. Thank you