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Linphone was found to be a working software to to the calls for rC3.
Go for the latest version 4.2.5 as an Appimage: https://www.linphone.org/releases/linux/app/Linphone-4.2.5.AppImage.
Make the file executable (chmod +x Linphone-4.2.5.AppImage) and execute it (./Linphone-4.2.5.AppImage). It will ask you for your username (the AuthUser from sip_configuration_hints), a displayname, the server and the password. Approve and give it a try.
Under Preferences → Calls and Chat, “Encryption” needs to be set to “None”.