To connect a Gigaset DESK 800A phone with a Jabra headset, you can use the built-in RJ-9 headset port on the phone. This will allow you to plug in a professional headset like the Jabra to keep your hands free during calls. Make sure that the Jabra headset is compatible with the RJ-9 port. For use with a Swisscom router, there are no particular restrictions on connecting several analogue telephone sets, but check that your router supports several analogue connections.