I am coming into this late in the game. It's easy to start any opnion with "it all depends on ___"...but it does. It depends on what kind of organization this is, what size it is, how strong it has been been and will be, what industry, what dynamics and what kind of leadership (which includes what kinds of values are embraced and replicated).
For some companies, 2-3 yrs is a gold mine for retaining call center staff. Some companies transition employees out of a customer service environment 6, 12 or 18 months after start date, if they are the kind of org that wants to bring in eager individuals with stamina and patience to move into other areas of the company, and presumably the exposure to the "entry-level" environment has a broad enough range of experiences to be gained, that this kind of patience and understanding of what is delivered by a company can be valuable training for other positions in the company. "Customer service" as a career skill in that kind of company may not be a marketable competency, but rather a short-term means to deliver for a higher objective
Other companies would have a different kind of culture if they specialize in a product or market and are large enough to have their own developed customer service department. There, the customer service dept might be more of a competency in its own right, and the "customer service you get at ____" is part of the company's image as well as its product/service. Maybe pay levels increase with months or years of service and there is room for growth inside the CS dept.
Still other companies are there to handle the customer service requirements of others, and THAT is their business.
But with each of those types of companies I tried to describe, there is still going to be variation in culture and leadership and how customer service is valued as a competency. My point of view is that the up or down levels of call center/customer service staff retention is going to be all about those cultural variables and all about why people are brought into a company to begin with and whether thought is given to what will be done with them on a regular basis.
So I'm not really answering the original question directly, but I guess the answers are behind the sketching I have done.