Is the call to commence prayer (Iqamah) an obligation or a Sunnah? What is its ruling in congregational prayer and individual prayer? And is it necessary to have two calls to commence prayer or one call when combining Maghrib and Isha prayers during a shortened prayer?

The Iqamah is a Sunnah for both the congregational prayer and the individual prayer. It is recommended when combining two prayers that share a common time, for each of them. If two prayers are combined at one time, it is recommended to say the Adhan for the first and the Iqamah for the second. However, if they are combined with a single Iqamah, there is no harm in that.