Possible cause and solutions
If your phone system has an answering machine and you see the
handset indicator flashing, it simply means that you have a new
message recorded on your answering machine.
It is also possible that your line is subscribed to a voice
mail service from your phone company and your handset is
alerting you that your voice mailbox just received a new
message.
As soon as you have listened to your new messages the handset
indicator should automatically turn off the indicator light.