To determine if a cellphone is unlocked, try powering down the phone, removing the SIM card, inserting an active SIM card from another carrier and turning the phone back on. Locked phones give a message saying that they are restricted or to contact the carrier.
As of 2015, carriers are willing to unlock most phones that are paid-in-full or off-contract. Owners contact their carrier who either sends a signal to the phone unlocking it or provides instructions for the user to unlock the phone. Some phones require the customer come into the store for unlocking.
The advantage of an unlocked cellphone is that the customer is able to choose another carrier. T-mobile offers a replacement SIM card that replaces the existing card in the user's phone to allow access to its network. Many other GSM phone networks do the same.
If consumers are switching to a new network that uses the same type of phone, an unlocked device potentially saves them hundreds of dollars that they would otherwise spend on a new phone. They also can switch carriers without signing up for a new contract. However, not all phones work on all networks. Unlocked AT&T phones do not work with Sprint because the two networks are incompatible.