question

I would like clarification on the missed calls in our call logs. About a quarter to a third of total calls are being classified as "missed" for two of our group lines  and some of them show a duration of nearly a minute. How is that possible if the system should be kicking people to voicemail after 20 seconds? Both lines in question are set up to ring everyone simultaneously. For the five or six second long missed calls, I'm assuming those callers just hung up the phone.