VBS: Business Rule of SG Phone Number Validation

Summary:

When trying to add SG phone number from Me → Contact Info, if the number of digits of phone number are less than 8, we get below error:

When checking under business rule, I am unable to see a business rule for this validation.

Can anyone pls confirm if this is a standard validation or a custom one and how can I see it?

Content (please ensure you mask any confidential information):

When trying to add SG phone number from Me → Contact Info, if the number of digits of phone number are less than 8, we get below error:

When checking under business rule, I am unable to see a business rule for this validation.