In theory what should happen if both phones are plugged in and idle is that ext 1100 will ring when a call arrives at the hunt group. If the call is unanswered by the time the No Answer timer expires, then the call will stop ringing 1100 and start ringing 1131. When the No Answer timer expires again, the call will start ringing 1100 again. If the hunt group has a Recall destination then the call will be transferred when the Recall timer expires. If 1100 is busy, then the next call will ring 1131 right away.

If 1131 is unplugged then it is not available. Therefore a call to this group will ring 1100 until the Recall timer expires (if there is a Recall destination programmed). If 1100 is busy the call will 'Camp-On' to the group and notify 1100 by both providing a short ring and an LCD update. There are station flags that determine whether the station hears a ring or not.
Also the above assumes hunt group defaults have not been changed.