Bilal would call the Adhan, then he used come to wait. When he would see that the prophet ﷺ had come out (of his house), he would pronounce the iqama.  

كَانَ بِلاَلٌ يُؤَذِّنُ ثُمَّ يُمْهِلُ فَإِذَا رَأَى النَّبِيَّ ﷺ قَدْ خَرَجَ أَقَامَ الصَّلاَةَ