If you aren't receiving Video and/or Audio in a call, or the far end are saying they the same about you, the chances are it is your firewall. Virgin Media seem to be the most common ISP with this issue.

To fix this you will need to login to your router and add the relevant firewall rules which can be found here

The Virgin media (domestic) instructions for all Hubs are here:-

https://www.virginmedia.com/help/virgin-media-set-up-port-forwarding-port-triggering

Virgin Media Business:-

https://www.virginmediabusiness.co.uk/help-and-advice/products-and-services/hitron-router-guide/hitron-how-to-set-up-port-forwarding/