ISDN not dropping Internet connection

We have an MS SBS 4.5 server running Proxy Server 2 connected to Demon via an USRobotics ISDN modem attached to the server's serial port. Exchange v5.5 also using this connection with scheduled times to call to retrieve mail..
We are experiencing times when the line stays up for hours at a time even when all PCs on the network are switched off. Time out is set to 2 minutes inactivity.

 NetBeui is not running as a protocol - only TCP/IP and IPX. There is a Novell server on the network as well.
Latest USR drivers are installed.
We had no problem with exactly the same set up using an Analogue phone.
Any ideas on what is causing the line to stay active ?
jteeceAsked:
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gjahchanCommented:
You first have to identify which application is causing the modem not to disconnect.

Stop all proxy services from MMC and Services applet in Control panel and test the mail server on its own. Does it call and hang up per its set schedule?

Temporarily stop the Exchange Server IMC & NNTP services and test the proxy server on its own. Does it hang up after a spec'd period of inactivity?

If not, check that LCP extensions are disabled in your phonebook entry.

Add the following line to the phonebook (rasphone.pbk) in the appropriate connection:

IdleDisconnectSeconds=120

If IPX is not required, disable it.

Hope this helps.

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