Element Call is an open source end-to-end encrypted video and voice conferencing solution built on the Matrix protocol for secure communication.
We’re developing Element Call to provide the best possible security properties with compulsory end-to-end encryption, supporting sender verification, forward secrecy, post-compromise secrecy, zero-trust decentralisation and cross-domain capability. It achieves this by building on the foundations of Matrix as a mature, audited [1], open standard protocol for secure communication.
That isn’t built on Matrix afaik, it’s specific to the Element client.