[zaik]

> Do not know of a single XMPP client that requires a phone number

Here is the only one I know: https://quicksy.im/

[infosechandbook]

Yes, Quicksy from the Conversations developer who bragged about copying main features of WhatsApp and Signal.

Another one is Kontalk, https://www.kontalk.org/.

Both XMPP clients require a phone number, and both present the phone number as a benefit in comparison with other XMPP clients without this requirement.

[zaik]

It's a trade-off. If you are unwilling to provide a phone number you can choose any other XMPP provider and still can communicate with friends who use Quicksy/Kontalk. You simply don't have this freedom if you use WhatsApp or Signal.