The ringing is a programmable feature. First you need to know the extension number of the phone. Then you go into Centralized Telephone Programming, and turn the ring back on for each line on the extension.
If you figure out the extension number, go to Extension 10 and do this:
Feature
Dial 00
Left Intercom Twice
Right Intercom Once
Dial the Extension Number to be programmed
Each line will light up. The lights will "flutter", as it has been programmed not to ring.
Touch each line button and the light will switch to a steady green. This indicates that the line will Ring on an Incoming call.
If you touch a line button again, the light will blink at an even, steady rate. This indicated that the line will ring on an Incoming Call after 4 rings.
Once you set each line to steady (immediate ring),
Feature
Dial 00