I should start of by saying that the company I work for is a very large Avay dealer and the largest ShoreTel dealer. So I feel I know both systems pretty well
The short answer is yes ShoreTel has solutions for large call centers. Do they rival Avaya, well yes and no. Avaya is the king of all phone systems when it comes to call centers, but with Avaya having the best is going to cost you a lot of money in the form of training, requires a trained Avaya expert to make changes, and is based on old but good programming language.
Now ShoreTel has a great solution, the basic call center package that comes included is ShoreTes is great for a lot of peopl, then their to two more flavors, contact center, and enterprise contact center (ECC). ECC is great and we have several very large call centers that use it, and the cost of ECC sompared to a Definity is much lower. The problem with the ShoreTel package, and the reason you will hear it criticized is because ShoreTel licenses the call center software from a company based in Israel, because of this you have to get support directly from the company and since Israel is about a 12 hour differnce, when you have that call center emergency and you cant get support on the spot it can be a real pain. If it is your job to support the system and a call center is down, the customer does not like to hear that you have to way several hours to get support.
So because of these problems ShoreTel has been trying to buy exclusive rights to the software but is having problems with the price. If they can't get rights to this software they are going to either partner with a new company (I have heard a couple of really good names thrown around) or build their own from ground up.
I am expecting great things from ShoreTel! I am a Cisco CCVP, Avaya certed, and have done 3 Com, Merdian, and a couple others and ShoreTel is the best enough that I have shifted my career to be around the product. Good luck and feel free to ask me any questions!