This article explains the format requirement for the phone number input field when the Profile Enrollment form Input field has a phone validation method enforced, as documented in the Okta Identity Engine (OIE): Create the custom profile enrollment form.

When a new end user attempts to perform self-service sign up and enter an invalid phone number, the self-service profile enrollment may fail with the Okta Sign up form error:

Enter a valid phone number.

• Profile Enrollment Form (OIE only)
• User Lifecycle Management
• Form input fields for phone number and has a Phone input validator enabled
• Okta Identity Engine (OIE)

The out-of-the-box user.mobilePhone attribute does not have any input format constraint by itself. However, if the Okta Admin configure to add user.mobilePhone in Profile enrollment form > Form Input configuration along with added phone input form validation method as well, then Okta will be enforcing an Phone number input field validation against Strict E.164 standard. 
  

Using the above provided sample problematic phone number: 22345678901:

• Missing +: If "Phone" validation is enabled in Okta, it often fails numbers like 22345678901 because it does not know which country 22 belongs to without the +.

• Strict E.164: Without the +, the system might be trying to validate it as a local number for the default region (likely US), and 11 digits would be an invalid length for a US domestic number.

To avoid the Okta sign-up form error when phone number validator is enforced, the end user is recommended to always enter a phone number that meets the official ITU E.164 recommendation, the format must:

• Use decimal integers
• Be limited to a total of 15 digits in length
• Start with a +
• Exclude 0 as the first character, as no E.164 country code starts with 0

The following diagram shows the E.164 format:
 

Country codes have a length between one and three digits. The combined length of the National Destination Code (NDC), also known as an area code, and Subscriber Number equals 15 minus the length of the country code.

Using the above sample sign up form input error, the phone number input validation is completed as soon as the end user enters a phone number begin with + character for example, +22345678901.