Ringback detection

Hello, everyone.
Could anyone explain how does ringback detection works in asterisk.
Sometimes, when making a call, my asterisk box doesn’t detect a ringback
and I just hear silence until the other party picks up the phone. I’ve
checked the SIP messages and they are ok (I’m getting 183 “session in
progress”), so I guess I should be debugging the RTP packets. From then
on I’m stuck. Does anyone know what type of packets I should be looking