All you need to do to keep your SIM card and US phone number active is use Roam's service once a year.
To keep your SIM and phone number active, all you need is to have a plan scheduled in the future - 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 and 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 you do not schedule a plan in the future or use the service at least once each year, your SIM and phone number will expire.