As someone else pointed out, you are very unclear about your telephone connections, and "role." I'll assume you mean you have rollover set up with the telco. In that case, you need two outside lines, each connected its own modem. When one line isbusy, the telco will roll the call over to the other line.
You don't need either the PDC or BDC to provide RAS. In fact, you would be better off using some other PC on the network, even an old 486, to perform this service. You can administer the RAS from the PDC.
The RAS PC must be in the domain. The Client that dials in to the RAS PC must have a workgroup name that is the same as the domain name. This sounds kind of strange, but it is very important. Keep in mind the remote Client cannot be a member of the domain until after he authenticates. But the Client must be a member of a workgroup that has the same name as the domiain into which he is dialing. You won't configure the DUN client to belong to a domain, you will only configure him to belong to a workgroup.
When you configure your RAS Devices on the RAS PC (even if it is the PDC), your are configuring 2 separate devices. Each one gets added in the Remote Access Setup one at a time. Then, when you administer theseRAS devices from your PDC Remote Access Admin utility, you will see two separate ports.
Don't forget to grant the DUN user dialin permission from the Users | Permissions menu in the Remote Access Admin utility on the PDC. Naturally the user would have to be a domain user first. And it wouldn't hurt to make the DUN user a local user on the RAS PC with dialin permission.
I recommend you run WINS on the PDC. It's simple to set up, and less hassle than trying to use lmhosts. Make sure yourRAS PC points to the WINS Server, as configured in Network, TCP/IP Properties, WINS Address tab.
Let me know a little more about your telephone line connections.
