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.
A Key Performance Indicator (KPI) is a specific measure of an organization’s performance in an area of its business. It is a general concept, with different implementations depending on the type of business and goals of the organization.
From the perspective of balance scorecard, KPIs can be categorized by financial, customer, internal processes and learning&growth.