I Have one incomming CO that does not light or ring and voice mail never picks up. Any ideas? Users also complain of dead lines when they try to call out. I have busyed-out the CO that is RNA but after that I'm lost. I have a manual but need some direction.

Before you get too crazy with manuals and programming, have you verified that you have a good line coming into the DK280?
Can you hook up a single-line phone ahead of the box? This may take some creativity depending on whether your COs come in on 2-line cords or convert from a 25 pair terminal block to the CO inputs thru an "octopus" cable.
I have 8 2-line jacks for 16 lines, so if I suspected problems with line 6 I would unplug the cord for 5/6 and use a 2 line adapter to plug my standard phone into the L2 side.
In the 6 or 8 years I've had my DK280 I have had 3 or 4 cases where I had to call the phone co. (Bell/Verizon) for loss of an incoming line. I have never had any problems with the DK280.

Thanks JKNichols. I have checked for a good CO by connectting a test set to the 66 block and the trunk is good. I also busyed-out all off the CO's one by one while calling the main number to establish witch line was RNA. It rings in at the block where I can answer it, but CO 15 is never recognised by the system. I also swaped cards to verify if it is a hardware issue. The trouble stayed with the trunk.

Have you tried using #7 plus the trunk numbers to see if you can get dialtone? Check programs 81-87 and 16 too.

Look at the LED on the CO card to see if it flashes when a call is coming in on that line. Check the Mod plug and make sure you have dial tone there (like JN said) Then check program 40 to make sure the voicemail ports have access to that line. If the ports don't have access in prog. 40, you'll get RNA.