I see no one has replied to you yet.  Go into the managers settings under phone then make sure that the HUD it turned on.  I bet you that for some reason it got turned off.  Why, I do not know.

Hey Stanley,

Thank you so much for your correspondence. I may have a little trouble seeing, or perhaps I do not have the role to see this setting (not the Super Admin).

Would you be able to link or show a screenshot showing this setting? This would so genuinely help

There you go. 

I am guessing somebody removed the digital line from the team manager, thereby rendering them unable to call external numbers and to monitor calls. 

If the team manager is able to call external numbers I am wrong. I suggest the team manager goes in the app > Settings > Phone > Edit extension settings (pic1 below)

This will open a tab in your browser. 
Then go to Settings > Phones & devices. 
If you see the message “No phones are available” (pic2) this means I am right and the team manager doesn’t have a digital line/RingEX license assigned.


 

By the way, did you add the manager to the call monitoring group after deleting their extension. If they have a digital line and are unable to monitor a call, the only possible reason is that they are not in the call monitoring group.