Next step is pull all the other extension and cot cards and see if that card works on it's own, It could be another card pulling it down, so if it works, add the others back one at a time and see which card causes it to stop! Remember that whilst the first eight slots (00 to 07) are the main slots you use, there is a correlation between slots 04 & 08, 05 & 09, 06 & 10 and 07 & 11 so if the problem card is in one of these slots, check if there is anything in the associated slot as the second 4 lens on each card also appear on the higher slot number. So if a card has been inserted in the higher slot and it isn't one that is allowed in there (AP cards such as ISDN cards or AP00 cards) that can cause a problem!