If your carrier supports it, you can pay for your iTunes Store, App Store, and iBooks Store purchases and your Apple Music membership through mobile phone billing.

Mobile phone billing is currently available with select carriers in Belgium, Germany, Japan, Norway, Russia, Saudi Arabia, Switzerland, Taiwan, and the United Arab Emirates.

On your iPhone, iPad, or iPod touch

1. Open the iTunes Store or App Store app.
2. Tap View Apple ID. You might need to sign in.
   
3. Tap Payment Information.
4. Under the list of payment options, select Mobile Phone.
   
5. If the phone you're using is the one connected to the carrier plan that you want to bill to, select "Use This Mobile Number.” Then tap next.
   If you don't see "Use This Mobile Number" or you want to use a different number, tap "Use a Different Mobile Number, " then . If you're not using iOS 8.4 or later, you'll need to .
6. Apple will use the mobile number of the iPhone you're on to check with your carrier and confirm that you can be signed up for mobile phone billing. You might see a screen that says “Verifying” while this happens.

If you don’t see Use This Mobile Number, or want to use a different number

If you're on a different phone, or on an iPad or iPod touch, you might want to use a different number to set up mobile phone billing. Follow steps 1 through 6 above, then follow these steps on an iPhone:

1. After you tap “Use a Different Mobile Number, ” enter the mobile number for the phone connected to the account that you want to bill to.
   
2. 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. If you don’t receive the code immediately, tap “Resend Code” to try again.
   
3. Tap Verify to verify the code.

On your Mac

1. Open iTunes.
2. If you’re not signed in, sign in with your Apple ID.
   
3. 
4. Enter your password, then press the Return or Enter key on your keyboard, or click View Account.

On the Account Information page:

1. Select Mobile Phone.
   
2. Enter the mobile number for the phone connected to the carrier plan that you want to bill to, then click Send Code.
3. An SMS message with a one-time code will be sent to the mobile number that you provided. Check the mobile phone for the message, and enter the code on the computer that you’re using to set up your payment method. If you don’t receive a code immediately, tap “Resend Code” to try again.
   
4. Click Verify to verify the code.

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Source: support.apple.com