Darren,
If the problem is indeed with increased call volumes and not, as Linda rightly points out, with rep behaviours, then you need to look at the types of calls you are receiving. You have to ask yourself why customers call you in the first place and identify if anything has happened in your industry or business that could contribute to increased calling. If you have such tools, you could investigate CTI or IVR records to highlight an increase in a certain type of call.
The solution also depends on your definition of call volumes. If you are referring to calls answered increasing, then follow the steps above. If your calls answered is the same but abandons are increasing, then the issue may be with not have the right number of reps in place at the right times (i.e. a scheduling issue). If your calls answered are decreasing & abandons increasing, then your issue is likely to be in rep behaviour, as Linda has alluded to.
Cheers,
Chris