At the time of this writing, 250 free voice minutes are automatically added to your account each month. If you run out of minutes before the end of the month, you can buy additional minutes to get you through the month. You can also pay for any extra services not included such as texting or international calls. You can pay for those services at a variety of locations (depending on your service provider) using credit, debit or PayPal.
Inversely, some companies offer rollover plans so that if you do not use all of your allotted minutes, you can roll them over to the next month. I you really don’t have much spare cash to buy more minutes, and need the the phone for emergencies, make sure to remember this phone is a lifeline service so don’t rely on it for social conversations.