I have done further tests and finally given up to get linphone working as a SIP client here - the segfault on password enty is absolutely reproducible.
To solve this issue I have tried "twinkle" (another SIP client from Buster-repo and that was set up within a few minutes. Just entered the SIP and STUN servers and my credentials for my t-online account - and all works as expected. This definitely confirms that it is not related to t-online, nor to some missconfiguration of my system. Moreover such a segfault is caused by severe bugs in the application itself. Again the syslog message: tux kernel: [ 4866.131680] linphone[6107]: segfault at 0 ip 0000003153a8f37a sp 00007ffe572604c8 error 4 in libc-2.24.so[3153a00000+194000] Mar 3 18:17:33 tux kernel: [ 4866.131690] Code: 89 e6 5b 4c 89 ef 5d 41 5c 41 5d e9 e0 49 ff ff 89 f8 31 d2 66 0f ef ff 09 f0 25 ff 0f 00 00 3d c0 0f 00 00 0f 8f 78 02 00 00 <f3> 0f 6f 0f f3 0f 6f 06 66 0f 74 c1 66 0f da c1 66 0f ef c9 66 0f of which the tail is always the same: ------------------------------------- ... 89 f8 31 d2 66 0f ef ff 09 f0 25 ff 0f 00 00 3d c0 0f 00 00 0f 8f 78 02 00 00 <f3> 0f 6f 0f f3 0f 6f 06 66 0f 74 c1 66 0f da c1 66 0f ef c9 66 0f There is also no means to gather more info about the crash in libc6 besides the syslog entry. Linphone's debug output. Even at highest level does not disclose anything valuable for me. So linphone 3.12 from Buster is buggy and absolutely unusable for me, severity should be set back to 'serious' to reflect that.
