Voxer - Turn your phone into a Walkie Talkie. Don't waste time on phone calls and voicemails.
Yac - Take your time back from Zoom & Slack
NextPlane - NextPlane offers cloud-based, scalable unified communications federation service for the enterprise.
Beseda - Async video and audio messaging for remote teams
Sameroom - Chat interoperability-as-a-service
Slack - A messaging app for teams who see through the Earth!