Network Chat Program

We have a NT 4.0 network with about 60 users all running Win98. Large majority of users are salespeople – who spend many hours each day on the phone. Am looking for suggestions/recommendations on some type of chat type program that would allow salespeople to relay information via there computers to other salespeople while they are are not phone. Am not looking at a chat room. Network is all TCP/IP with DCHP assigning our IP address.

Thanks for the help
Jeff