1- The correct view is that adhan is fard kifayah (a communal obligation); if someone who is competent does it, the rest are absolved of sin. 2- If a person starts to pray without the adhan or iqamah, whether out of forgetfulness or ignorance, or for some other reason, the prayer is still valid. 3- If a muadhdhin gives the adhan in one neighborhood and all the residents can hear it, that is sufficient. For more, see the detailed answer.

The adhan and iqamah are only for the obligatory prayers. But for Tarawih, Witr and other optional (nafl) prayers, there is no adhan or iqamah.

Two forms of the iqamah have been narrated from Prophet Muhammad (peaceand blessings be upon him); one form has 11 phrases and the other has 17 phrases.