@xiaofan sorry I should have asked my question about SIM-only plan here.
so based on your reply, no need to unsubscribe
Auto-Subscribe to ReadyRoam. However, I worry that if it remains enabled, and I land my flight overseas that doesn't come with a bundled data (e.g. USA), it will automatically subscribe (as the wordings imply) to the local telco (e.g. Verizon) and become chargeable, even if I am not using their data or calling while roaming.
The word 'auto' seems to imply that it will be active on its own.