There are different KPIs or Key Performance Indicators that you can check for the grade of service. One of them will be total number of calls handled ; but it is not the most accurate one. Look for AHT , ASA and AWT indicators.
Grade of service is very numerical; I will give more weight to customer satisfaction reviews and sting reviews.
You can search for a properiatay method called ECHO for inbound processes.
I hope this helps.