Here goes - (hope I don't forget anything!)
1. You can use one phone line, but if you are using the same line for RAS dial-in and dialling out to the Internet, you may have some problems (i.e., one or the other won't be able to connect when the other is using the line). However, if it's only one user and only occasionally, you should be OK.
2. Right click on Network Neighborhood and click Properties. Click the Server tab. If you already have a dial-out connection set up on the server, the Remote Access Server item will already be there. If not, you need to click the Add button and add it from the list of services that will appear. (You'll have to reboot - and be sure to re-apply the latest NT service pack afterwards as well.)
3. If it's already there, highlight it and click Properties. Highlight the modem and click Configure - make sure it's configured to allow both dial in and out.
4. Click the Network tab and set up your TCP/IP properties for dial-in users. You can either let your DHCP server assign IP addresses, or you can create a static IP address pool for RAS users. Make sure to enable access to the entire network, if they need to browse to resources on other hosts on the network (other than the SBS machine).
5. Finish out your settings by clicking Continue and/or OK wherever applicable.
6. One other thing you need to check is to be sure that the bindings on your server are maintained in the correct order. Before clicking the last OK on the Network Properties box, click the Bindings tab and check to be sure that the Remote Access WAN Wrapper is listed AFTER your internal and/or any external NICs for all services.
7. Once you finish all these settings and click OK on Network Properties, you'llbe forced to reboot.