Hi Matt,
Well, really the ultimate choice will be up to you as to whether calls should be entered as cases or activities. But, hopefully I can shed some light on the situation for you to make things clearer.
An Activity is an individual task that someone does, such as recording an email received, or making an outgoing phone call. You can log these in individually and track them by user or you can view them in the Sales Center or Call Center dashboard, etc.
A Case is one or more activities that are related to a particular outstanding issue, concern, or problem that a customer is trying to have resolved. A case would be opened for this customer and all activites that are performed in trying to resolve this issue can be related back to that particular instance via the Case Number. Then, once resolved, you can post a Solution for the issue so that others who view the record can see how the situation was resolved for that customer.
I used to work at a call center as an inbound call rep a few years ago. We took inbound calls for an auto manufacturer. Calls would range from compliments (although VERY RARE!), to warranty issues, to MY CAR'S A LEMON complaints (those were always the fun ones!). For every inbound call we received, we created a new case and entered a new activity right away for that case. The activity reflected the inbound call that we received. Now that case could be tracked in a variety of ways, and provided other reps with details of what transpired on the call. We used to have to sometimes deliver some bad news to the customer and it was commonplace for disgruntled customers to call up again in hopes to receive a different rep who might give them a different answer! No such luck for them because once saved, any rep could pull up a case and see everything that was going on and everything that was already done for that customer.
We were evaluated on our "One Call Resolution" which was a goal in which we strived to resolve most customer complaints in just one call, preventing the need for us or the customer to have to make another call before closing the case. The call center manager can create a report based on cases that shows him/her the average number of calls it takes for a customer's issue to be resolved. The manager can then use this data to try and enhance the 'one call resolution' procedures already in place.
So, really it's up to you how you should enter inbound calls. Hopefully I've made the differences between Activities and Cases clearer for you.