i huge recommend both SIP + XMPP software :)

we’re not classic, we’re advanced nerds engineers : signal is one church without any interoperability, so we’ve moved to both SIP and XMPP variants using differents provider in the family : everything is decentralized, exactly like email (we dont use gafam ones). It’s not that hard to enjoy xmpp, and really decentralized tools, a bit like for SIP (we have several Voip providers here).