When I was kid (many, many year ago), I learned multiple times that when one is praying the silent Shemoneh Esrei and the Chazzan's repetition is up to kedusha, one can answer Kadosh and Baruch kevod only. My Rebbi in 9th grade even read from the Mishnah Brura this fact, but it was not in the regular area. The regular area said that one can only answer this in Elocai Netzar with yimloch
If you have heard of this, can you please give the the source of this? Yashar Koach for your help.