Open your store's Shopify admin > Checkout settings page.
Search for Customer contact section, and select the option that allows customers to checkout using "phone number or email", as shown in the screenshot below:
If you need to select the email-only option, it's ok. Make sure to complete Step 2.
Search for Form options section on the same page.
Set Shipping address phone number as Required or Optional. We recommend setting it to Required as it helps to collect more phone numbers for opt-in.
We recommend setting it to Required if you selected email-only option in Step 1. Otherwise, we won't get the customer's phone number to send WhatsApp message.
Search for Email Marketing section on the same page. Select both the checkboxes as shown in the screenshot below:
Selecting this checkbox helps you get maximum possible opt-ins from the Checkout page.
Click on the Manage checkout language button as shown in the screenshot below:
In the Manage checkout language page, search for Accept marketing checkbox label using Ctrl+F keys (CMD+F on Mac) as shown in the screenshot below:
Update the text field shown above to ANY of the following options:
Include any additional channels that you are using in the text above. For example, if you use SMS, the first message becomes "Get order updates via WhatsApp and/or SMS".
On the same page, search for Phone label or Optional phone label using Ctrl+F keys (CMD+F on Mac) as shown in the screenshot below:

Update the text field shown above to:
This will help communicate cleary to the customer about what the phone number will be used for, in case they use their email to checkout.
Don't forget to click the Save button on the top-right corner of the page after you're done!
Almost done! In case you missed any of the steps, you can click on the "Back" button and complete them.
If you completed all the steps successfully, your checkout page should look similar to this:
All done? Go ahead and click on "I have completed all the steps"