Local Users not allowed to dial in

I just set up a new software for an ordering system for a our clients
on a non production Dell Poweredge 2850 with Windows Server 2003. The
way the system works is a user dials into the server to authenticate,
get an update to the program and/or send an order through.

The server is part of our domain but we never actually log into the
domain to run the ordering program.
Our sales reps are a part of the domain and are added to the Users
Group in Computer Management.
Our customers are individually added to the Users group as well.

Our sales reps which authenticate through the domain can dial into our
server fine however our customers cannot dial into our server even
though they are a part of the users group.

They enter their username, password and domain as the local computers
name.
The error they are getting is as follows:
Error 691: Access was denied because the username and/or password was
invalid on the domain.

Any support is appreciated,
The Gopher