so audio always works in messenger (whatsapp, signal) calls? just as a backtest.
That a restart fixes things points to a subsystem crashing without recovery. An (adb) logcat at that moment could explain more when a call action will do nothing. Do you have the impression this was introduced with a specific /e/OS release?
It could be a bug/regression, not necessarily a 2G issue if the operator still does them (each country has a different phaseout year).
4G voice / VoLTE on Samsungs
Samsungs with official builds atm can only do 2G calls. If the operator starts to wind down 2G voice, the devices will be affected.
For some models there are experimental builds with ims, what enables volte / 4G calling. Afaik iodé bundles an open ims stack in their beta 7.7 builds for the S10 series. I’ve read positive feedback only for the S10e though, not S10.
For the S9 and others there are unofficial /e/OS builds in this forum with bundled ims.