Windows 95/NT Dial-Up Networking Issues

This may be a simple question, but it’s outside of my expertise.

We have several laptop users (95 and NT) who need to access our network from the road. We have an 800 number for this, and dialing in and connecting seems to be no problem. However, most people either get the No Domain Server error when prompted to log in to the network, or get no prompt at all, merely a TCP/IP address. All of the network settings are correct to log in, and WINS and DNS servers are specified. Any ideas?