Lines usually ring at the prime set as a last resort. The issue you are having can be an indication that something else in your system is not properly programmed or that a feature has been turned on. Here are some examples:
[ul]
[li]If a line is not programmed to ring any set on your system, it will ring at the prime set to alert that there is a call coming in.[/li]
[li]If a line is programmed to ring on one set and that set has DND turned on, it will ring at the prime set so someone else can answer the call.[/li]
[li]If a line is programmed to ring on some sets and none of them answer the call after a few rings, it will start ringing at the prime set.[/li]
[/ul]
In all cases, a message appears on the display of the prime set while it is ringing. What is your?