Unfortunately, I do not have a digital line card in my NBX100 that I have sitting here for testing. I used to work for a small company that serviced NBX systems though, so I'm still pretty familiar with them.
It sounds like your dial plan is not acknowledging that it needs to look at calls coming in on the new line card. Realize that the NBX is smart enough to know that the new line card is a different piece of hardware than the old one. Unfortunately, it is not smart enough to know that you want it to replace the old one.
Look over the settings under Dial Plan. I'm thinking the Extension Lists section may be key. Under Dial Plan-Operations, do an Export on your dial plan and look over it for clues.
I'm guessing you will be able to either make a change under Dial Plan - Operations, or you may be able to edit the dial plan with a text editor and import the modified plan to fix this.
You may also need to look in Device Configuration under Digital Line Cards to see if the old card is still listed, and compare its settings to the new cards settings.
I wish I wasn't so rusty on these things...it's been almost two years now since I have serviced these.
