Hello & Welcome to Just Answer,
May I ask:
1) If you (or Mother-in-law) owns this system already OR if you (or she) are considering purchasing this system?
I see it has been a few days and I just wanted to follow up to see if you have any more questions concerning this issue or if the issue was resolved.
I did check and found that this particular system DOES have an "Alert" setting - where it will flash an indicator on any handset in the system when a new message has been left on the system. It should be ON by default, but can also be turned off. If you have the full operating instructions for this system, refer to page 63 for details on how to program this setting.
I look forward to any further questions or results that you may have.
Thanks, ***** *****--. .-(im)