I've had this issue as well. I sent a message to Subaru and here is what I got back. Not super helpful, but at least they responded within a day and seemed fairly genuine although mostly canned:
Thank you for your email received by Subaru of America, Inc. We appreciate you taking the time to contact us.
Congratulations on the purchase of your 2012 Subaru WRX STI model. We very much appreciate that you chose the Subaru brand for your vehicle needs.
You may find the infroamtion listed on page 5-36 of your Owner's Manual helpful. There are very specific situations where it may be difficult for the individual you are calling with hands free to hear you clearly.
We would also recommend bringing your concerns for the bluetooth to the attention of your Subaru dealership. They can check the function of the unit, compare to other like models on their lot and determine whether there is an issue.
We will forward your comments regarding the bluetooth function/clarity to the Product Planning Department. Such information is reviewed and sometimes used when designing future products.
Again, thank you for taking the time to contact us. If you have future comments or questions, please contact us again.