Let me rephrase this. UNPLUG BOTH AMP CABLES from the cabinet.
If you have a "testar" plug it into one of the 2 amp jacks. Do you get a display? If you don't get a display in EITHER cable, look for a testing failure. A digital phone uses the 2nd of 3 pairs to work in either cable. Look around, swap the cables, move P2 into P1's slot and see if any of those phone work. Do whatever it takes to learn if you have a bad testing cable, a shorted cable somewhere, or a plain miswired tester.
If you don't have a tester and only one cable, pull all the bridge clips or make it so that you're only looking at the 2nd pair of the AMP cable (white-orange pair). You only want to look at the first or first two pairs with nothing else plugged into the extensions. If you don't get digital station dial tone with only one jack installed, then you probably have a cabinet problem.
You may find that once you clear the short on the extensons, the cabinet will start functioning.
LkEErie