I find it mystifying that they haven't fixed the problem in the new SL4. There's more than enough information in this thread to enable MS engineers to identify the issue. I don't understand why they haven't come up with some solution.
Pierre, I installed the free RingCentral Classic app (as recommended in some previous responses) and it seems to work for me. The application is launched on startup but can be closed immediately, though in Task Manager you can see it there as a background process. It takes a certain amount of memory, but CPU usage seems minimal and I'm hoping that the impact on performance and power consumption is also inconsequential.
It's crazy that this kind of workaround is necessary on a brand new high spec device, but on balance I find it preferable to starting again with a different supplier.