HEy Bill
Have your read
how to build a pbx The Asterisk pbx isn't a VOIP service, you will need a VOIP Service such as Broadwing or Vonage to send your calls over the internet.
The digium line card will allow you to use your Asterisk PBX and Sip phones and make calls through the analog line that you have attached.
The only VOIP calls you would be making would be a SIP to SIP call if you have 2 or more PC's with SIP phones on them. But when you try to make an outgoing call it would go over your analog line.
Does that make sense?