Repeated Dialup Attempts

Have 6 networked systems, 5 running XP Home and 1 XP Pro. 56k Dialup modem is on XP Home system and all other systems access the internet via Internet Connection Sharing through a gateway on that system.

For unknown reasons at inconsistent times (usually overnight when the office is unoccupied)
the modem attempts to connect automatically even though I have disabled all auto-update features in all applications.

Am I being hacked from outside??? or is there another explanation???