The people mentioned the fire and the bell (they suggested those as signals to indicate the starting of prayers), and by that they mentioned the Jews and the Christians. Then Bilal was ordered to pronounce Adhan for the prayer by saying its wordings twice, and for the Iqama (the call for the actual standing for the prayers in rows) by saying its wordings once. (Iqama is pronounced when the people are ready for the prayer).
Narrated Ibn `Umar
When the Muslims arrived at Medina, they used to assemble for the prayer, and used to guess the time for it. During those days, the practice of Adhan for the prayers had not been introduced yet. Once they discussed this problem regarding the call for prayer. Some people suggested the use of a bell like the Christians, others proposed a trumpet like the horn used by the Jews, but `Umar was the first to suggest that a man should call (the people) for the prayer; so Allah's Messenger (ﷺ) ordered Bilal to get up and pronounce the Adhan for prayers.