Finding the billing error is easy. Getting the credit? Not so much.
Claims for billing errors are rarely as simple as "the carrier is billing 3 cents per minute even though the contract says 2 cents per minute."
The root causes can depend on whether certain waivers apply, whether custom pricing is being correctly applied, or whether a particular discount applies in conjunction with another discount. If there's room for doubt, or the contract is less than crystal clear (as supplier-drafted contracts usually are), suppliers will push back on a recovery claim.
So it's important to present billing recovery claims with as much supporting information as possible, and to be prepared for the counter-arguments that will inevitably be made. Ultimately, the billing recovery process becomes as much a negotiation as a billing review. Being prepared for that and what it entails, including having the right resources on your team (i.e., negotiators, not just billing analysts), is key to the success of the project.
And once the process becomes a negotiation, expect suppliers to leverage all kinds of issues against your billing claims. That unrelated contract extension you are negotiating? Suddenly, the pricing concessions that had been offered "include" release of your billing claims, perhaps for pennies on the dollar. Those accounts receivable issues you've been working through with the supplier? Suddenly, the supplier will only address your billing errors if you pay up on all open amounts. You need to anticipate that and be ready for it