The minhag in this sfardi minyan is to daven mincha with the chazzan starting shmone esrei out loud and everyone also starting with him. Everyone says kedusha and then everyone continues queitly. What should an ashkenazi do in terms of morid hatal, nakdishach instead of nikadesh. Do you say leumasam and uvidivrei? if yes do you say leumasam mishabachim .. or luemosom baruch..? The chazzan says ata kadosh, do you also say ata kadosh or ldor vador?
Personally I’ve never heard of such a thing, that the whole tzibbur says the Shemona Esrei out loud, but assuming that this is based in halacha of minhag, What is done, and maybe it was confusing, is that the tzibbur remains quiet and answer kedusha, and after ato kadosh, they start thier Shemona Esrei.
The general rule regarding these things like R, S. Z. Auerbach zt”l said, that when a person is davening with a minyan that has a different nusach then his, that anything that is said out loud should be said like the minyan ‘s nusach in order not to appear to be davening differently then the tzibbur. However anything that is said quietly, is said according to his nusach.
Halichos Shlomo (Tefillah) 5 ftnt. 31.