Multitech has a lot of good products that may do something similar to what you are desiring. I doubt that they would give you handsfree between sites - but they can tie the sites together over the internet. I have not done this on Avaya, but have with Comdial DX-80 and it works fine for the customer to call between sites, but not to extensions, it basically takes an analog extension and rings into a CO line on the other end over the public internet.