Shameless plug. AirSend (https://www.airsend.io) offers the bare essentials one need to collaborate with both internal team members & external clients in one elegant package. Collaborating across organization boundaries is really hard with both ms teams and slack. Thats the problem we are trying to solve with airsend.
AirSend gives you a few simple (chat, voice/video, files, tasks and wiki) but powerful tools and then get out of the way.