Question: At Birkat Kohanim, who says the phrase, “Am k’doshecha ka’amur”?
Answer: The Rema (Orach Chayim 128:10) rules that the shliach tzibur should recite this phrase silently. The custom to do so is cited by the Aruch HaShulchan (128:18). The Mishnah Berurah (128:36) explicitly notes that the shliach tzibur should say this phrase and writes that the custom of kohanim to recite this phrase is improper.
Nitai Gavriel (Hilchot Birkat Kohanim p.63, note 11) reports that a number of siddurim contend that the congregation say “Am k’doshecha ka’amur” aloud. He cites an Acharon who rules that both the shliach tzibur and the kohanim should say it. In addition, he cites the Manhigai Frankfort (amud 24) who states that the congregation says only “Am k’dosehcha” but not “ka’amur.”
Based on all the above, it is understandable why Birkat Kohanim in large shuls with many kohanim generally manifest confusion.