Must the muezzin repeat the call to prayer, or is that limited to those who hear it?

It is sunnah for the one who hears the adhan to repeat the words of the muezzin, except for "Hayya 'ala as-Salah" (Hasten to the prayer) and "Hayya 'ala al-Falah" (Hasten to salvation), for which he should say "La hawla wa la quwwata illa billah" (There is no might nor power except with Allah). This is based on the Prophet's (peace be upon him) saying: "When you hear the muezzin, say as he says." This applies specifically to the listener, not the muezzin.