I know how to create both of these items but what I cannot get past is what makes Business Process Testing different from Call To Test? To me it seems that I can define business processes as template tests and string calls to these together to create a test.
BPT is more geared for automation. It sets up the framework for non-technical people, like BA's who know the industry and business processes. I use the model, but not the actual BPT module for my automation framework. At its core is a design efficiency that allows BA's to create all the processes and then submit them for automation to the automation engineers. Then they can just take the small automated components and build larger business processes, like end-to-end flow, like Hire-to-fire scenario.
It is popular at places that have more BA's than QA's, or where BA's do all the testing.
Using Calls to other tests, is similar in concept, but BPT is a more evolved solution with tighter integration and process built in.
There is a HP white paper on their site. "HP Business Process Testing software: test automation focused on your business"