All of a sudden one of our remote branch office Option 11's cannot establish a D-Channel connection to our Main Office CS1000 via a Cisco PRI. This has worked fine ever since I started working here 5 years ago but all of a sudden on Thursday its failed. I have replaced the DTI card in the remote Option 11, rolled the DTI card in the Option 11 and have verified that the cards are good. I have checked the cabling from our Cisco 7206 in the main office to the PRI card in the Option 11, checked the cabling from the PRI card in the Option 11 to the WIC card in our Cisco 2801 at the remote site. We have swapped ports on the 7206 and verified that the Cisco is not the issue. We are not seeing any alarms and if I do a show voice call sum in the 2801 router it looks as if 8 B-Channels will establish but not the D-Channel. Our maintenance company is saying it is Cisco, our Cisco vendor is saying it's Nortel. Any ideas of what else I can look at?