Explanation/Reference:
Explanation:
The question states that "You need to prevent all call center personnel from communicating with external contacts by using Lync Online, while still allowing other employees to communicate with external contacts." Therefore, we need to apply the restriction to the call center personnel only.
The question also states that "They still must be able to communicate with internal users." You can turn off external messaging for a single user by clearing the People on Public IM Networks check box. This will override the company setting of allowing external access. The image below shows the People on Public IM Networks check box.
To ensure that the call center personnel can continue to be able to communicate with internal users, we need to leave the Lync users checkbox ticked.