Hi,
I am trying IVM myself, and it's new too me.
Just figuring out about the use of variables, but here's what I would try:
You could add a line to a special log file every time a voter guessed correct and arrived at the OGM at the end of the correct path. Then count the number of lines in your text file.
Or you could write a number in the text file. The first success is number 1. When the next successful caller calls, you read the number from the textfile, add 1, and write the new number in the text file. I don't know how IVM handles multiple successful callers at the same time in case you have multiple lines, maybe you'll miss a count in this case (??).
For this to work you have to use the plugins Getdata and savedata. Or SQLLink (for use with a database).
Sorry, no exact answer, but I hope it helps.
If you happen to know if there is a built-in variable "call number" that counts the incoming calls, please let me know! 
-Martijn