In the days of Prophet Muhammad (d.) azan was called from the gate of the mosque. In the days of Khulafa-e Rashideen (Radiallahu taala anhu) the same rule was followed. Azan had been never called from inside the precincts of mosque in the days of Prophet Muhammad (d.) as well as Khulafa-e Rashideen (Ra.).
Hadith/ Hadis Shareef:
حَدَّثَنَا النُّفَيْلِىْ ثَنَا مُحَمَّدُ بْنُ سَلْمَةَ عَنْ مُحَمَّدِبْنِ إسْحَاقَ عَنِ الزُّهْرِىْ عَنِ السَّائِبْ بِنْ يَزِيْدِ رَضِىَ اللهُ تَعَالى عَنْهُ قَالَ: كَانَ يُؤَذَّنُ بَيْنَ يَدَىْ رَسُوْلِ اللهِ صَلَّى اللهُ تَعَالى عَلَيْهِ وَ آلِه وَ سَلَّمَ إذَا جَلَسَ عَلَى الْمِنْبَرِ يَوْمَ الْجُمُعَةِ عَلى بَابِ الْمَسْجِدِ وَ أبِىْ بَكْرٍ وَ عُمَرَ
According to a Hadith/ Hadis reported by Daood ascribing to Saeb bin Yazeed (Ra.) through Zohree, Mohammad bin Ishhaq, Mohammad bin Salmah and Nufailee.
On Friday when Prophet Muhammad (d.) used to come on pulpit, azan was called at the gate of the mosque in front of him (before khutbah). Same system continued in the days of Hazrat Abu Bakr Siddeeque, Hazrat Farooq-e Azam (Ra.).
(Quoted by Ala Hazrat in Aufal lumahfee Azan-e yaumil Jumah from Sunan of Abu Daood I/156)
This Hadith/ Hadis proves that custom of calling azan from the gate of mosque was present right from the days of Prophet Muhammad (d.) and Hazrat Abu Bakr Siddeeque (Ra.).