Office has 6 Meridian phones (M5312), six lines. Each phone able to access all lines, but each line rings only at phone assigned to that line. In the last week, two of the six have ceased ringing on the line assigned to them, hence calls are coming in but we do not hear them. Is there something I can do to fix this myself?

5312's,are you leasing service from your local telco?If you are,then you need to call them.