Quoting Alf <alf.debian...@gmx.de>:
This definitely confirms that it is not related to t-online, nor to some missconfiguration of my system.
Still, it would be interesting what leads to that behaviour. I use Linphone as my one and only telephone and it works fine for me on both a stretch and a buster system, both amd64. No crashes at all. I use different providers, however.