Baseplans renew every 30 days. If you've registered for automatic top-ups, the baseplan will automatically renew. Talk and data booster add-ons do not expire. However, once you've used up all of the minutes in a talk booster, talk boosters do not auto-renew. The same applies for data boosters once you've exhausted the data allotment.
When you login to your prepaid self serve account, check the Overview page and look under the heading; Current Booster Add-on(s). Click on the button; Add Booster Add-on(s). If you don't use your phone much for talking, select the $5 - 25 anytime minutes’ option. This will allow you to retrieve your voicemail messages.