This could be due to Firewall protection of your network. Usually, most of the corporate networks block certain ports on their Firewall for security issues.
Video Conferencing uses multiple TCP and UDP ports. Some organisations don't have a Firewall that can handle H.323 (Video Conferencing), or some ports may be purposefully blocked. If this is the case, you should only have an audio connection.
You can contact your IT Team to unblock the ports needed for you