I also happened to work in a very huge C++ project for the business logic component of a call center, using CORBA and the Reactor pattern from ACE to integrate different parts of the user / ACD / 1st agent / specialist / CTI combo.

But it was a telco project, so the "let's use a FSM" was well thought from the beginning. Turns out you could easily know what the system would do just by reading a huge Smartdraw document, and this also allowed for reasoning on new use case scenarios. It did not matter that the main file was a horrendous 40kLOC C++ class because the Smartdraw diagram was the documentation and everything else was just trivial code.