Ras w/ Exchange

I currently have a full time dsl line that gives everyone on the network access to the internet. We also run exchange 5.5 and I want to enable remote users to use offline folders and be able to synchronize remotely. So I think I need to implement ras on the exchange server, but do I have to use an actual phone line connected to the exchange server, or can they connect to their ISP and then connect to our office (IP address) through the dsl to synchronize and how would I go about setting itup this way? Any help would be greatly appreciated.