Through the OSI model w/modem connection

I am pretty sure that when someone dialups to a ISP port the modem call begins going through the first 3 layers of the OSI model (dialtone, call placed, connection established, modem negotiation speed = physical layer, datalink layer, and network layer). What i am most interested in is what layers protocol establishement and radius authentication takes place. I have a feeling that happens on the transport and session layers of the OSI model. The answer to this question would be most helpful in troubbleshooting incompleted calls to Lucent Maz TNT switches.