The red light indicates that it has some news, whether that is no contact with base, time not set, no dial tone, or missed calls. If you have multiple hadsets you may have to check the others depending on the problem. To find out what the news is, simply pick up the phone that is flashing and press the OK buttom and it will display for example 'missed calls' then press OK and cycle through the missed call list, once you have done this the light will stop flashing.
@JOFF the Beocom 6000 will flash if calls are missed, automatically, it is not a setting you have to apply unless you have disabled it in the first place.