Hello @emajcic,
the above URLs need to be accessible from the client where Outlook and the Webex Scheduler Add-In are installed. Depending on your Exchange environment (on-premises or M365) and how you deploy the Webex Scheduler Add-In (either from Microsoft AppSource or using the manifest file) you will need access to either the Microsoft AppSource URLs or the Webex URL for the manifest file (https://meetings-api.webex.com/wbxaddin/normal/manifest_official.xml) from your Exchange on-premises environment (if used).
I haven't seen a communication flow for the Webex Scheduler Add-In, so I cannot provide any details regarding this. To my knowledge, the Webex Scheduler Add-In communicates to Exchange through EWS and via HTTPS towards Webex and above listed services.
Best regards
Igor