MS Exchange IMS with ISDN

I have a client that just installed a Netopia R3100-U ISDN router for Internet access on their network backbone. Their ISP is providing the ISDN access via a leased IP address (DHCP???).

I would like to configure their MS Exchange v5.5 server with IMS/IMC to login to an ISP “mailbag” using ISDN for all Internet mail for the client’s domain. I have done this using RAS and a modem previously, but do not know how to do it via ISDN. Is the scenario even possible?

Any help would be killer-diller!!!