Tree or Server not found

We have a number of sales staff that work from home and need to login to our NetWare 4.11 server over a phone line. They have Dell laptops with NT Workstation and the Novell client for NT (a mixture: 4.6 SP2, 4.71 and some 4.8). They dial-in via a Shiva box. A lot of them have been getting ‘Tree or Server not found’ errors. I have searched the Novell Support pages but none of the TIDs have helped. Does anyone have any ideas? Is there anyone else who has similar problems??