Webex Teleconference services for the site http://gunungsewu.webex.com should be enabled.
For scheduling Webex via TMS to work, it requires Teleconference service not just VoIP. Because, TMS scheduling uses CMR Hybrid feature and this feature requires Teleconference. The information is documented at https://help.webex.com/en-us/rtcj4e/Known-Issues-and-Limitations-for-Site-Administration and it says “CMR Hybrid meetings do not support "VoIP Only" audio”. However, when you schedule a meeting directly on the Webex site, it uses the normal meeting service (not CMR Hybrid), hence it works fine.