Question about FRITZ! Fon C5 International / CH

I ordered and received a Fritzfon C5. Now I see that I can't use it at all. I thought that I could just plug the cable into the router (modem) like I did with my current Dect phone and it would work like it did with my current one. But apparently that's not the case, I need more equipment and I don't want that. Can I return the C5 and order another Dect phone instead? Kind regards Peter Robert Mattenweg 7 4528 Zuchwil perowa@gmail.com