Cell Phone Auctions How Do You Unlock A Cell Phone To Sell It To Someone Else For Free Use?

How do you unlock a cell phone to sell it to someone else for free use? - cell phone auctions

I think the online auction from my phone Cingular Motorola RAZR Black achieving buy a new phone. I want to ensure that the buyer has no problem with the phone, and I guarantee you, they do not reach the same void. When I unlock the phone, it is necessary that the buyer also use Cingular or they can use the service of their choice?