Are you hearing ringing or just seeing the line appearance light when the call comes in. Have the lines been programmed to be answered by a UCD??

Where did you get this system?? Do you have the manual? Who did the initial programming? Have that person check out the C.O. Line ringing program.

How big of a system is this?? Under 4 lines--under 8 stations?? Do you have voice mail with it??

You might need some on-site help with this from a qualified technician. By default---extension 101 rings on all the lines. If it's not, at least, doing that---then there are some problems that may not be able to be solved on this forum. May need someone there to check things out.