If there is an on call DL and the active person is removed from the system through and AD delete, causing that DL to have no one actively "on-call" it would good to have a notification pop up or message sent to the user letting them know that there isn'y anyone there.
Through testing, we know that if an on call DL has no one listed, the user can't actually send a message to that DL>
The propblem comes in when a user sends to a Group with multiple members and one of those members is an "on-call" DL
Ex:
Message to "Rapid Response Team"
-The Rapid Response Team consists of: 
-Surgeon On-Call
-Respiratory Therapists
-Nurses
-Techs
-Etc.

In this scenario there is no one listed as "on-call" for the Surgeon On-Call DL because they were systematically removed and the sender has no indication that a Surgeon isn't coming.