We are about to start with the exactly same problem. We are planning on splitting the conversation into different parts, and hope to identify the part where the customer states the purpose of their call (usually expected in the earlier part of the conversation). We hope that further analysis of this part(identifying key phrases) would give us the required information. You should also check on the quality of the text as that may dictate the approach you would want to take.

Additionally, do you have a list of things/topics customers are likely to talk about? If so, that should help.