I posted this months ago and it is still not fixed (and the support was suboptimal, to say the least):
When I have an ongoing call on one SIM and a new call comes in for the other SIM then this call does not show up and is also not listed as missed call. The calling person hears a few peeps and then the line is cut like when you reject the call.
This is a serious problem.
I tried the default phone app and also the simple phone app from the f-droid store: both show the same problem.
It does not matter whether I use e-SIM or the physical SIM for the first call.
(unfortunately the original thread is closed now)