Outlook client dialling every 2 minutes

I have a remote Win XP PC running Outlook 2002 using dial up ISDN over a WAN to connect to Head Office, where there are 2 domain controllers and an Exchange 5.5 server.
If I leave the PC on at Ctrl Alt Del prompt the line drops after 60 seconds of inactivity, as per router config.
If, however, I go into Outlook 2002 then do nothing the line again drops after 60 seconds of inactivity BUT comes up again every 2 minutes!
I am currently testing to see if the other 2 remote PCs at the same site (running Office 2000) cause the same problem, but I don’t believe that’s the case.
Any clues?
Regards
Adrian Raine