This function is available by default when you purchase any Data Roam plans. This is how Data Roam Network Lock works:
1) Protects you from unintended pay-per-use data roaming charges on non-preferred network when you are on Data Roam plans. Data roaming will resume once you are connected back on a preferred network.
2) When you are in a destination not covered by your existing Data Roam plan, your data access will be blocked. You may subscribe to another roaming plan available in that destination.
3) When you are in a destination not covered by any of our Data Roam plans, your data access will be blocked.