What metrics to use when evaluating the work of the Call-center?

Question

Call centers

The question is narrower. Interested in quality metrics. There is no need to write about the items below
% of missed calls
Time on hold
Number of calls received by each operator
Number of calls per day, etc.

Answer 1

Customer happiness/satisfaction

Answer 2

Entire books are written about it.
I can show, tell and set everything up on a commercial basis.
Built and maintained call centers with 400k+ calls per month.