What is more effective, to do the NPS calls by the managers or the Frontlines from engagement perspective? Should we open a CLF for each Detractor?

If we are responsible for the creation of detractor, then with no other option we should open CLF, to identify the root cause, correct to prevent the creation. 

Basically we must make sure the customer engagement is positive! That alone ensure employees and customers to participate in the rating for NPS.

The most effective one is the NPS calls by the managers because the CLP can be done by frontliners, to me is a requirement to be done to all clients regardless of their origion and management when they do NPS it will make client to feel valuable and secure.