As long as you use Roam’s service once per year, your SIM card will remain active and you’ll keep the same US phone number. After 365 days of inactivity, both will expire.
To keep your SIM and phone number active, all you need is to have a plan scheduled - you do not need to travel to keep your number active. If you’re not planning to travel anytime soon, remember that you can schedule a plan to start up to 6 months in the future, then go back and reschedule it later.
When scheduling a plan to keep your SIM active, you should purchase the plan you usually use, or what you think you'll use on your next trip. This way, when you know the dates of your next trip, you can reschedule your previously purchased plan for your trip without spending extra money on a new plan.
If your SIM card expires, you will need to purchase a new one in order to use Roam’s service again. After you activate online and purchase a plan, upon entering the US, your new phone number will be sent to you via text message or email.