After updating my Linux system, which included fontconfig 2.13.91, zoiper no longer works because it seems to use its own fontconfig which can no longer read the new config files.
$ zoiper
APP Started! [ PID: 58648 ]
APP Started! [ PID: 58650 ]
APP Started! [ PID: 58669 ]
[1004/151542.130668:WARNING:browser_main_loop.cc(335)] Gtk: Unable to locate theme engine in module_path: "adwaita",
[1004/151542.133126:WARNING:browser_main_loop.cc(335)] Gtk: Unable to locate theme engine in module_path: "adwaita",
Fontconfig warning: "/etc/fonts/fonts.conf", line 5: unknown element "its:rules"
Fontconfig warning: "/etc/fonts/fonts.conf", line 6: unknown element "its:translateRule"
Fontconfig error: "/etc/fonts/fonts.conf", line 6: invalid attribute 'translate'
Fontconfig error: "/etc/fonts/fonts.conf", line 6: invalid attribute 'selector'
Fontconfig error: "/etc/fonts/fonts.conf", line 7: invalid attribute 'xmlns:its'
Fontconfig error: "/etc/fonts/fonts.conf", line 7: invalid attribute 'version'
Fontconfig warning: "/etc/fonts/fonts.conf", line 9: unknown element "description"
Fontconfig warning: "/etc/fonts/conf.d/10-hinting-slight.conf", line 4: unknown element "its:rules"
Fontconfig warning: "/etc/fonts/conf.d/10-hinting-slight.conf", line 5: unknown element "its:translateRule"
(... etc ...)
Fontconfig error: Cannot load config file from /etc/fonts/fonts.conf
Trace/breakpoint trap (core dumped)
