Thanks for the link didn't spot that one during my last Microsoft knowledge base search marathon... I reckon the info in the article is along the right lines for a lot of "random dial" problems.
I set up a test rig last night with two Win2k Pro boxes connected to a LAN with an Ethernet printer. The only devices on the network were the two Win2k machines and the printer.
Machine A has an internal 56k modem setup for ICS and a 3Com NIC set to use the default IP address supplied by ICS.
Whenever either machine tried to print the dial up was triggered.
Doing an ipconfig/all from the cmd prompt showed that both Win2k machines were set as Node type = Hybrid. This means that NetBIOS uses a mixed mode of name resolution and my guess is that in Hybrid mode when a machine wants to access a network resource it tries to use NetBIOS name service ( NBNS ) which triggers a dial up ( A bit like DNS forwarding where the DNS server propagates unknown requests to another DNS server out on the web).
A reason why a machine triggers an ICS connection when it boots could be that when the computer starts it tries a NetBIOS name lookup ( NBNS ) which fires a connection via ICS.
So, I searched the MS knowledge base for a way of changing the machine node type and found the following two articles...
Win2k / NT
When I set both machines to a B node type ( Broadcast ) the printer dialling problem went away.
Also tried this with Win Me computer and it also worked.
Will need to test this theory a little more but hopefully it might be an answer to some strange dialling problems