We are a prepaid carrier, which means you pay up-front for 30 days. After 30 days of service, you can renew the plan you had or pick a new one. Your renewal date is 30 days from when your line is activated. If you're transferring a number to US Mobile, then your plan begins on the day that your port completes.
To help remind you when your line expires, our dashboard displays the number of days till your line expires, we email you three billing reminders and even let you set up Auto Pay which automatically renews your line every 30 days (with whichever plan you choose).