The Preferred Roaming List on a Metro PCS allows you to connect to the networks in the list. This way, if your phone's connection to one network is weak, it can connect to one of the other networks in the list that has a stronger connection. If you want to update this list as more networks become available, there is a specific code you must dial on your phone's keypad to do so.
Dial "*228" on the keypad while in your local calling area.
Choose option 2 from the list of options that appears.
Follow the on-screen prompts.