Too long dialing time

[Igor Zbirka]

I'm using FreeBSD 3.4-RELEASE as a server which should provide connection to internet through dialing on demand (tun0).
The connection to internet works fine, but the problem I have is with disconnecting.
If one of the computers in LAN wants to send for ex. email, the server dials and establish connection. But then it keeps that connection even though the email was send long time a go.
According to the led's on modem I can see that sporadicaly there are still some datas send and received. But WHY?

Because of this sporadical datas being send and received the limit 60 seconds for disconecting the modem don't arrive even for 12 hours. And this 12 hours was started by sending one email.
I would like to know how can I check what is being send and from wich computer in my LAN.
Any ideas how?

Thanks.

Igor Zbika