: You can buy airtime directly through the carrier's app (e.g., the My Verizon or My AT&T app) or their official website.

: Many carriers let you dial a code like *BAL# or #646# in the Phone app to receive a text message with your remaining balance.

: You can purchase "Skype Credit" to call landlines or mobile numbers globally.

If you don't want to deal with a traditional carrier, you can buy minutes through third-party apps that use your iPhone’s data or Wi-Fi:

: Most convenience stores and big-box retailers sell physical "Refill Cards." You enter the PIN from the card into your iPhone's phone dialer or the carrier's website to activate the minutes.