Since you're on an iPhone or iPad, Apple handles in-app purchases for 2nd Phone Number subscriptions. This means you can use any payment method associated with your Apple ID:
Credit cards (Visa, MasterCard, American Express)
Debit cards
Apple Pay (if available on your device)
Apple Account Funds or Balance
Want to see all the accepted options? Here's the deal:
Payment methods can vary depending on your location. To see exactly what's accepted in your country or region, visit Apple's website: https://support.apple.com/en-us/111741
Here are some additional points to remember:
Prepaid options aren't available yet: Currently, the 2nd Phone Number App doesn't accept prepaid gift cards or phone credit for direct payments.
Stay updated: For the latest and most accurate information on accepted payment methods, it's always a good idea to contact the 2nd Phone Number App's support team directly. They'll have the most recent info for you!