One sad thing about SIP that despite being pretty common here in Czechia, it is only used as a last hop to PSTN instead of as an independent federated network.

One sad thing about SIP that despite being pretty common here in Czechia, it is only used as a last hop to PSTN instead of as an independent federated network. It makes economic sense - it is hard to monetize running SIP servers for independent network (and one cannot use ads like with e-mail as SIP clients are not web apps), but you can monetize selling access to PSTN. Today, with WebRTC, one can build web client for SIP, but WebRTC VoIP services are still just silos.