Solution:
Currently, there is no option available for a Site Administrator to change the default call-in numbers on a Webex website, with Webex Events (Classic) as the only service type.  To get the default call-in numbers changed, please talk to your company’s Webex Site Administrator or Cisco Support. See:  WBX15 – How do I get Help with Webex?

On a site with Webex Events (Classic & New) + All Services (Webex Meetings, Webex Training, and Webex Support), changing 'default call-in numbers' in the Site Admin tool will affect Webex Meetings and Webex Training only.