Chances are your phone bill lists all 4 outside numbers. When there's no traffic (so line won't roll over) dial each of the numbers. The two that don't answer are the ones you need to find. Look at the device from your carrier...can you find those two lines? If so, trace the connections...do they connect to your phone system? If the carrier is delivering the lines on regular jacks switch the ones that work with the ones that don't...if problem stays with the number, it's carrier issue but if problem moves to the other numbers the problem is upstream in either wiring or programming. Assuming your system is the HX chassis, about 3.5 " high, your phone lines should plug into the ports labeled LS 1-2 and LS 3-4 on the back of the cabinet, 2 lines per plug. If it's an old CS chassis, about 1.75" high, 2 lines will appear on a single LS jack on the chassis, the next two on a single port 1-2 on an LSM module.
Probably!
