PCs call Remote Server via ISDN dial up

Windows 2000 PCs (SP3) call the Head office Servers seemingly of their own accord several times an hour, even at login prompt.
They connect to an ISDN router via a hub.
The 3 servers are running Win2K (latest service packs) also and are 2 controllers and an Exchange file/print server.
PCs – Netbios is disabled over TCP/IP, as is ‘Register this connections settings in DNS’.
The Microsoft auto-update service is turned off.
The PCs work locally and/or connect the network domain to pick up email etc.
Tel bills are subsequently high, even though PCs are turned off at night.
The rogue calls seem to be using protocols 1 or 17
(ICMP or UDP).
How can I stop these calls please?