It is possible you have a bad CSU but I doubt it.  99% of the time it is the circuit (although they always blame your equipment).  Are all your channels being used for voice?  Do you break of any with your CSU?  If they are all voice, just remove the CSU from the loop and plug the PBX directly into the smartjack.  You are supposed to have a CSU but it will work without.  

Check your T-1 card in your switch as well and all cables.  If you have a spare, swap that out too.  If all this done and it still doesn't work, go back to AT&T.  Its probably their problem anyways.

Ken

98Converter...You had your answer the minute you saw the red alarm on the NIU (smartjack). If the circuit is LD, then open a ticket with your Carrier (AT&T). If local, open the ticket with your LEC. It is true that they always try and blame your equipment (they are usually buying time to fix the real issue), but another thing you can do while your waiting is to do some loopback testing. Most all CSU's have loopback capabilites. You can place a loopback from the CSU towards your CPE (PBX). Look to see that your T1 card goes green. This proves your PBX is and the drop side of the CSU are both good. Now you can place a loopback plug in the Network port of the CSU. If the PBX goes green, then you can feel comfortable that the CSU and PBX are good. Only thing left is the cable from the CSU to the NIU as far as you're concerned. Everything else TELCO's responcibility. Good hunting.

Excellent advice. I will take this into the future with me. We did find the problem, it was corroded wiring from AT&T...