If you're on a different phone, or using an iPad or iPod touch, you might want to use a different number when you set up mobile phone billing. 

When you change your payment method, tap 'Use a Different Mobile Number' and enter the mobile number for the phone connected to the account that you want to bill to.

An SMS message with a one-time code is sent to the mobile number that you provided. Check the mobile phone for the message and enter the code on the device that you're using to set up your payment method, then tap 'Verify' to verify the code. If you don’t receive the code immediately, tap "Resend Code" to try again.