During the time of the Prophet ﷺ and Abu Bakr and 'Umar the call to the Friday prayer was first made at the time when the imam was seated on the pulpit (for giving the sermon). When the time of 'Uthman came, and the people became abundant, 'Uthman ordered to make a third call to the Friday prayer. It was made on al-Zaura' (a house in Medina). The rule of action continued to the same effect.  

أَنَّ الأَذَانَ كَانَ أَوَّلُهُ حِينَ يَجْلِسُ الإِمَامُ عَلَى الْمِنْبَرِ يَوْمَ الْجُمُعَةِ فِي عَهْدِ النَّبِيِّ ﷺ وَأَبِي بَكْرٍ وَعُمَرَ ؓ فَلَمَّا كَانَ خِلاَفَةُ عُثْمَانَ وَكَثُرَ النَّاسُ أَمَرَ عُثْمَانُ يَوْمَ الْجُمُعَةِ بِالأَذَانِ الثَّالِثِ فَأُذِّنَ بِهِ عَلَى الزَّوْرَاءِ فَثَبَتَ الأَمْرُ عَلَى ذَلِكَ  

The call to the (Friday) prayer was made at the gate of the mosque in front of the Messenger of Allah ﷺ when he sat on the pulpit, and of Abu Bakr and 'Umar. The narrator then repeated the same tradition as reported by Yunus.  

كَانَ يُؤَذَّنُ بَيْنَ يَدَىْ رَسُولِ اللَّهِ ﷺ إِذَا جَلَسَ عَلَى الْمِنْبَرِ يَوْمَ الْجُمُعَةِ عَلَى بَابِ الْمَسْجِدِ وَأَبِي بَكْرٍ وَعُمَرَ ثُمَّ سَاقَ نَحْوَ حَدِيثِ يُونُسَ  

There was no other mu'adhdhin (pronouncer) of the Messenger of Allah ﷺ except Bilal. The narrator then reported the tradition to the same effect.  

لَمْ يَكُنْ لِرَسُولِ اللَّهِ ﷺ إِلاَّ مُؤَذِّنٌ وَاحِدٌ بِلاَلٌ ثُمَّ ذَكَرَ مَعْنَاهُ  

There was no other mu'adhdhin of the Messenger of Allah ﷺ. He then narrated the tradition which is incomplete.  

أَخْبَرَهُ قَالَ وَلَمْ يَكُنْ لِرَسُولِ اللَّهِ ﷺ غَيْرُ مُؤَذِّنٍ وَاحِدٍ وَسَاقَ هَذَا الْحَدِيثَ وَلَيْسَ بِتَمَامِهِ