This phone number is only populated if the customer checks out using a phone number during checkout, opts in to SMS notifications, or if the merchant has manually entered it.
You can add a condition to your account page:
{%- if customer.phone != blank -%} {{ customer.phone }}
{%- else -%} Phone number is not filled in.
{%- endif -%}
To allow the user to enter the phone number on the registration page you need to change the field for entering the phone number
Once registered, the phone number will be recorded in the user’s “notes” in the admin panel.
However, in order to display this phone number on the account page it must be filled in anyway, either by the administrator or by the user when the user is on the checkout page.
Unfortunately I don’t believe it is currently possible to collect a phone number directly using the default registration form, unless you only store it in the notes section as someone else mentioned.
Another option would be to use an app like Helium Customer Fields to create a custom registration form which would allow you to collect things like phone number, address, or other custom fields, and have them saved directly to the customer’s account.