Question about Gigaset CL660HX handset (CH version)

Only with Salt wifi6, but not with Salt wifi7

Unfortunately, I could not find any information from Salt that the Salt fibre box has an integrated DECT base. I would also recommend contacting Salt directly. Otherwise you will need a base:
https://www.galaxus.ch/de/s1/product/gigaset-box-100-im-version-schwarz-telefon-65355116