If your phone is locked, your option is limited. A Locked Phone with one network provider won't accept the SIM card of some other network provider. In some cases, for example, you can use Cricket Wireless on locked AT&T phone because Cricket is on ATT network, or use Ultra Mobile with locked T-Mobile phone because Ultra Mobile is on T-Mobile network, but it's not guarantee and you are taking the risks on your own.  It is always advisable to get your phone unlocked and enjoy all the services and freedom of choices.