Exchange/Outlook won’t work on ISDN WAN

We have a Wide area network with all servers centrally. We have ISDN-Bri (128K) connections between our central location and 3 small offices. For our central office, we selected Outlook and Exchange to handle group calendars, individual calendars,and email (potentially, tasks, other Intranet functions, etc). We would now like to include the branch offices in our system — Is there some way to make this happen?
One person told me Citrix would be an answer. Another told us to shift to Novell groupwise. How can we get these branches to be part of our system — is more bandwidth an answer, where is the cost/benefit, from your experience?