
Purchase IMEI unlock codes for the T-Mobile E110 Zest to allow it to be used on all networks.
A network lock, otherwise known as a SIM lock, is a control placed on the phone by the original network that sold it. They are able to do this because with the lock, they can then sell the phone for cheaper, offering the customer a better upfront deal. This lock does however stay with the phone permanently unless it is removed with an unlock code or software. Doing this is completely legal and won't impact your warranty in any way.