Ask HN: Modern (open-source) VoIP/SIP client

There are many startups that provide cloud-phonesystems, some of them do also have webrtc-implementations right now. Some do have a own voip/sip client, but alomst any of the clients do lack in usability, UI (sometimes very old), stability, maintenance and/or cross-platform support.

I am looking for a SIP-Client for Windows which does simple provide a "just-working" calling-feature in a modern and stable UI / UX. It would be great, if the same tool is provided for iOS with call-kit integration. Open-source would be an awesome bonus.

I am happy to pay for the software, but just can't find one. Do you know one?