[Serusers] newbie - Handle incoming calls?

Hi All, I'm new to all this stuff, so apologize if these are dumb questions. I have an account with a local/ld voip provider. They provide me a SIP connection to/from the pstn. I have this working with sjphone and xten. I also have ser running (very basically) and handling/routing calls for local softphones registering with it. I really want to be able to receive incoming pstn calls and route them internally. Here's where my ignorance comes in. The sjphone and xten register at the vendors registrar as sip:nnnn@vendor.com. I havn't seen an example or read anything how I could configure ser to do that. Also, the SIP line is authenticated. How do I tell ser to use a set of credentials when sending/receiving SIP messages to my vendor? Thanks! Tim