the sx-50 has 10 slots. your trunk card is in slot 8 (alarm reads 08-04). the trunk you're having trouble with is the 4th trunk on that card. take the cover off the switch, and follow the cable from card 8 to the 66 block. follow the jumper from trunk 4 to telco interface and isolate it from the pbx and test it for dt, reversal, whatever.