If you don't know your traffic, you might as well tape a "kick me" sign on your backside
Would you blindly accept the balance shown on a restaurant check without thinking about what you ordered?
If you are sourcing telecom services and leave it to the carriers to determine the call volumes and circuit quantities in their proposals, you are in basically saying “the waiter is always right.”
Carriers may select a sample month with unusually high volumes that, when annualized, overstate savings. They may omit or misclassify international terminating call volumes or include MPLS ports that you plan to disconnect in a month. To maintain control of the procurement process, you have to “know your traffic.” This means developing and providing an explicit inventory of minutes and circuits to the providers and asking them to price that inventory, not what they wish you had.
The inventory should be a representative sample and when you run proposals against it you need to factor in anticipated growth or decline in demand because of known business or technology changes. That will make savings projections more accurate because you will be able to compare current and proposed costs (as well as multiple bids from different suppliers) on an “apples-to-apples” basis.