In this article, we'll walk you through how to add an alternate phone number to your Apple ID on your iPhone so that you can use that number in Messages and other apps.
Steps

Step 1. Launch the Settings app
Click on the gray gears icon. It is located on one of the desktops or in the application bar.
If this icon is not in the application bar or on any of the desktops, look for it in the Applications folder

Step 2. Tap iCloud
It's at the top of the fourth section (along with the iTunes & App Store and Wallet & Apple Pay options).
If you haven't signed in to iCloud on iPhone yet, enter your Apple ID username and password

Step 3. Tap your Apple ID
This is the first option to include your name and primary email address.
You may need to enter your Apple ID password

Step 4. Tap Contact Information
This is the first option in the second section.

Step 5. Click Add Email or Phone Number
This is the last option in the first section.

Step 6. Tap Phone Number
Make sure the checkbox is next to Phone Number and not Email Address

Step 7. Click Next
You will find this option in the upper right corner.

Step 8. Enter the phone number you want to add to Apple ID

Step 9. Tap Next
You will find this option in the upper right corner.

Step 10. Confirm your phone number with the code
A confirmation code will be sent to the entered phone number

Step 11. Enter the verification code
The phone number will appear in the Contact Information menu as a verified number.
- The phone number you entered will not replace the primary email address on your Apple ID - it will simply be added to your Apple ID.
- Also, the phone number entered will be associated with your iMessage account.