This may be caused because the number field being used is associated with a custom_number field in the Luminate Online database. Custom_number fields have a limit to the size of the number that can be accepted. Due to this limit, they are not meant to be used to store phone number data. Instead, Contact Support and reference this article.  and ask them if there is another available custom field that can be used for this purpose. A support analyst may need to adjust some configurations on your site, to make a new custom field available for use.