I like the UI of Amazon Chime: https://chime.aws/

Though it is business focused.

1. There is a free plan, but limited.

2. Android, iPhone, Mac, and PC clients that work well.

Occasional notification syncing issues. That's about the worse of it.

3. I don't know; I haven't used Skype in years. Better than Lync and Polycom.

4. Yep.