RAS + WANs

What is the best way to achieve TCP conectivity with the following setup.

Domain in London, 2 way trust to Domain in Scotland. The servers only current connection is when the Scot server uses isdn to synch mail.

I need some users in Scot to be able to access the London network via ISDN.

Scotland server is both PDC/Exchange5.5 and RAS. The ras connections all use dhcp.

how can I get the Scotland server to make a isdn connection and allow full routing via this DHCP connection ?