The most important thing to know before searching for an inbound call center is the volume of calls you expect to receive. When figuring your expected volume out, try to be as accurate as possible—as hefty overage charges are often incurred if you go over your specified limit, and also because paying for unused service is unnecessary.
You can discuss seasonal and occasional call volume fluctuation with them so that they are aware of the service you need, and you get the coverage you want at the right price. Many inbound call centers can accommodate fluctuations with their pricing plans as long as it does not come as a surprise.