Most phones sold in North America — particularly on contract — are "bolted" to a specific cell transporter. They must be utilized on that transporter's system, so you can't change to another bearer without "opening" it first.
How to Unlock a Locked Cell Phone and Why You Might Want To do so
Most phones sold in North America — particularly on contract — are "bolted" to a specific cell transporter. They must be utilized on that transporter's system, so you can't change to another bearer without "opening" it first.