Number phones these are ordinary XMPP ID's +email@example.com, but no federal connection.
Maybe now is the time to join the XMPP open Federation? https://404.city/ or
In addition to WhatsApp, there are many other XMPP messengers with open source and federated communication. https://f-droid.org/en/packages/eu.siacs.conversations/
There are no physical problems for scaling the XMPP network, earlier in the 200's years the XMPP federated network was much larger
Federation is not beneficial for large companies, as it will cause a leaving users to small servers, #decentralization and the destruction of the monopoly
@icedquinn @Vulkan69 This source is outdated technical overview. It describes the state of the protocol for 2008 and now it is 2021.Difficult to read because obsolete extensions are mentioned everywhere (bosh, jingle, otr, dialback, Binary Data Transfer, etc).
Most of the issues discussed are no longer relevant, as the protocol changes over time.
XMPP already has video calls (Conversations, Dino)
XMPP already has file transfer (http upload)
XMPP network more popular OMEMO instead of OTR
To support this server and the OMN project https://opencollective.com/open-media-network