“There is no off the shelf software that would allow us to support calls of that size while ensuring that all communication is end-to-end encrypted, so we built our own open source Signal Calling Service to do the job.”
Mixing video on a server is easy, and it means the server sees everything. Signal’s server forwards encrypted streams it can’t read. Nothing off the shelf did that at scale, so Signal wrote its own and open sourced it. Zoom spent 2020 claiming end-to-end encryption it didn’t have.