You have to do the availability check so if it has never worked you have to go to menu then settings then caller id. The do check availability and if it says you are eligible you have to hit ok to get it started.
If its worked in the past and is no longer working then you have to disable the alerts. Reboot the box and then re-enable the alerts.
So menu>settings>caller id>alert display>disable Then reboot so menu>customer support>in home agent>reboot set top box (stb) Then re-enable alerts menu>settings>caller id>alert display>enable
If that doesn't work then open a support chat or call support. 888-553-1555