From my studies, the origins of altar calls are about 150 years old. Around the turn of the 20th century evangelist used the altar calls to count how many "salvations" occured.
It appears in the Book of Acts, the method was preach and preach and preach until something asked or something supernatural happened.
Acts 2, men asked Peter and the others what they needed to do, Peter stopped preaching after an outpouring of the Spirit at Cornelius's house. Paul preached on Mars Hill until someone said, I want to know more.
Just a thought. I'm not sold on the altar call method completely.