We mentioned previously that it says in Ben Ish Hai, that one must never omit the Hazarah (repetition of the 'Amidah) when praying, except in a case where one has no choice, such as where there is no time left.
(See http://www.atorahminute.com/2009-11-11 )
In a case where one has no choice, such as when there is no time left, The Rema states that the Hazzan can say the beginning of the 'Amidah out loud with the congregation reciting it quietly word for word with him, until HaE-l Haqadosh. However it is preferable for there to be at least one person answering Amen. The Mishnah Berurah states that the congregation, in such a case, should not start the 'Amidah till the Hazan finishes HaE-l Haqqadosh. This is the custom of Ashkenazim.
The Ben Ish Hai writes that in such a case (such as where there is no time left), the Shaliyah Sibbur (Hazan) should pray the first three Berakhoth (blessings) loudly together with the congregation who are reading it silently with him from the beginning, in order to say the Qedusha. After completing "Attah Qadosh", both the Hazzan and the congregation continue silently. This is the custom of Sephardim.
(See Rama 124: 2. Ben Ish Hai, yr 1, Parashath Terumah, Oth 2)
