Jeff,
Give me a department to call you at and I will try and reach you next week. We can go through how you are using your Centrex internally and externally. It is possible you could get by with as little as 55-75 trunks. There are many factors, such as call center agents, high call volume departments, ancillary use and so on. In some cases Internal traffic is the key, if it is heavy, a pbx can carry the load quite nicely. I have 2 customers that fit in the 1000 line area, I will check on their trunking and applications. This will at least give you a start. PRI and 2way DID could save a significant amount.
thanks bob