To add time to your Straight Talk phone, you must purchase a Straight Talk service card. Then you must add the card to your phone by going to the prepaid menu and selecting the Add Airtime option. You can also open the browser and select the option Refill My Account.
If your Straight Talk phone does not have either of these options, you can refill your account by going online to StraightTalk.com/Refill or contact the customer care center at 1-877-430-CELL (2355) for more support, as of 2015. To buy and add a Service Card for someone else, you can only add the Service Card online or by telephone.